ENH: refactor coordSet writers (#2347)

- the very old 'writer' class was fully stateless and always templated
  on an particular output type.

  This is now replaced with a 'coordSetWriter' with similar concepts
  as previously introduced for surface writers (#1206).

  - writers change from being a generic state-less set of routines to
    more properly conforming to the normal notion of a writer.

  - Parallel data is done *outside* of the writers, since they are used
    in a wide variety of contexts and the caller is currently still in
    a better position for deciding how to combine parallel data.

ENH: update sampleSets to sample on per-field basis (#2347)

- sample/write a field in a single step.

- support for 'sampleOnExecute' to obtain values at execution
  intervals without writing.

- support 'sets' input as a dictionary entry (as well as a list),
  which is similar to the changes for sampled-surface and permits use
  of changeDictionary to modify content.

- globalIndex for gather to reduce parallel communication, less code

- qualify the sampleSet results (properties) with the name of the set.
  The sample results were previously without a qualifier, which meant
  that only the last property value was actually saved (previous ones
  overwritten).

  For example,
  ```
    sample1
    {
        scalar
        {
            average(line,T) 349.96521;
            min(line,T)     349.9544281;
            max(line,T)     350;
            average(cells,T) 349.9854619;
            min(cells,T)    349.6589286;
            max(cells,T)    350.4967271;
            average(line,epsilon) 0.04947733869;
            min(line,epsilon) 0.04449639927;
            max(line,epsilon) 0.06452856475;
        }
        label
        {
            size(line,T)    79;
            size(cells,T)   1720;
            size(line,epsilon) 79;
        }
    }
  ```

ENH: update particleTracks application

- use globalIndex to manage original parcel addressing and
  for gathering. Simplify code by introducing a helper class,
  storing intermediate fields in hash tables instead of
  separate lists.

ADDITIONAL NOTES:

- the regionSizeDistribution largely retains separate writers since
  the utility of placing sum/dev/count for all fields into a single file
  is questionable.

- the streamline writing remains a "soft" upgrade, which means that
  scalar and vector fields are still collected a priori and not
  on-the-fly.  This is due to how the streamline infrastructure is
  currently handled (should be upgraded in the future).

src/meshTools/coordSet/writers/gnuplot/gnuplotCoordSetWriter.H

@@ -0,0 +1,179 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2011-2016 OpenFOAM Foundation
+    Copyright (C) 2021-2022 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Class
+    Foam::coordSetWriters::gnuplotWriter
+
+Description
+    A coordSet(s) writer in gnuplot format.
+
+    The formatOptions for gnuplot:
+    \table
+        Property    | Description                       | Required | Default
+        buffer      | Use buffered output               | no  | true
+        compression | Use file compression              | no  | false
+        precision   | Write precision in ascii          | no  | same as IOstream
+    \endtable
+
+    When called with a single coordSet, will buffer the output of
+    all fields and output together in the order of label/scalar/vector/...
+    each of which is sorted alphabetically according to the field name.
+
+    When called as a track writer (eg, with multiple coordSets),
+    will emit one file per field.
+
+SourceFiles
+    gnuplotCoordSetWriter.C
+    gnuplotCoordSetWriterImpl.C
+
+\*---------------------------------------------------------------------------*/
+
+#ifndef Foam_coordSetWriters_gnuplotWriter_H
+#define Foam_coordSetWriters_gnuplotWriter_H
+
+#include "coordSetWriter.H"
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+namespace Foam
+{
+namespace coordSetWriters
+{
+
+/*---------------------------------------------------------------------------*\
+                        Class gnuplotWriter Declaration
+\*---------------------------------------------------------------------------*/
+
+class gnuplotWriter
+:
+    public coordSetWriter
+{
+    // Private Data
+
+        //- Output stream option
+        IOstreamOption streamOpt_;
+
+        //- ASCII write precision
+        unsigned precision_;
+
+
+    // Private Member Functions
+
+        //- Templated write operation
+        template<class Type>
+        fileName writeTemplate
+        (
+            const word& fieldName,
+            const UPtrList<const Field<Type>>& fieldPtrs
+        );
+
+        //- Templated write operation
+        template<class Type>
+        fileName writeTemplate
+        (
+            const word& fieldName,    //!< Name of field
+            const Field<Type>& vals   //!< Local field values to write
+        );
+
+        //- Templated write operation
+        template<class Type>
+        fileName writeTemplate
+        (
+            const word& fieldName,
+            const List<Field<Type>>& fieldValues
+        );
+
+
+protected:
+
+    // Protected Member Functions
+
+        //- Write buffered data
+        virtual bool writeBuffered();
+
+
+public:
+
+    //- Runtime type information (no debug)
+    TypeNameNoDebug("gnuplot");
+
+
+    // Constructors
+
+        //- Default construct
+        gnuplotWriter();
+
+        //- Default construct with specified options
+        explicit gnuplotWriter(const dictionary& options);
+
+        //- Construct from components
+        gnuplotWriter
+        (
+            const coordSet& coords,
+            const fileName& outputPath,
+            const dictionary& options = dictionary()
+        );
+
+        //- Construct from components
+        gnuplotWriter
+        (
+            const UPtrList<coordSet>& tracks,
+            const fileName& outputPath,
+            const dictionary& options = dictionary()
+        );
+
+
+    //- Destructor. Calls close()
+    virtual ~gnuplotWriter();
+
+
+    // Member Functions
+
+        //- Enable/disable internal buffering
+        virtual bool buffering(const bool on);  // override
+
+        //- Characteristic output file name - information only
+        virtual fileName path() const;  // override
+
+        declareCoordSetWriterWriteMethod(label);
+        declareCoordSetWriterWriteMethod(scalar);
+        declareCoordSetWriterWriteMethod(vector);
+        declareCoordSetWriterWriteMethod(sphericalTensor);
+        declareCoordSetWriterWriteMethod(symmTensor);
+        declareCoordSetWriterWriteMethod(tensor);
+};
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+} // Endnamespace coordSetWriters
+} // End namespace Foam
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+#endif
+
+// ************************************************************************* //