ENH: Added a new function object to create a field relative to a field value

Calculates and outputs a field whose values are offset to a reference
    value obtained by sampling the field at a user-specified location.

    The field values are calculated using:

    \f[
        f_c = s(f_{c,t} - f_p + f_{off})
    \f]

    where
    \vartable
        f_c     | field values at cell
        s       | optional scale factor (default = 1)
        f_{c,t} | current field values at cell at this time
        f_p     | field value at position
        f_{off} | offset field value (default = 0)
    \endvartable

Usage
    Example of function object specification to calculate the reference
field:
    \verbatim
    pRef
    {
        type        reference;
        libs        ("libfieldFunctionObjects.so");
        ...
        field       p;
        result      pRef;
        position    (0 0 0);
        scale       1.2;
        offset      100000;
    }
    \endverbatim

src/functionObjects/field/reference/reference.H

@@ -0,0 +1,180 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2018 OpenCFD Ltd.
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+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::functionObjects::reference
+
+Group
+    grpFieldFunctionObjects
+
+Description
+    Calculates and outputs a field whose values are offset to a reference
+    value obtained by sampling the field at a user-specified location.
+
+    The field values are calculated using:
+
+    \f[
+        f_c = s(f_{c,t} - f_p + f_{off})
+    \f]
+
+    where
+    \vartable
+        f_c     | field values at cell
+        s       | optional scale factor (default = 1)
+        f_{c,t} | current field values at cell at this time
+        f_p     | field value at position
+        f_{off} | offset field value (default = 0)
+    \endvartable
+
+Usage
+    Example of function object specification to calculate the reference field:
+    \verbatim
+    pRef
+    {
+        type        reference;
+        libs        ("libfieldFunctionObjects.so");
+        ...
+        field       p;
+        result      pRef;
+        position    (0 0 0);
+        scale       1.2;
+        offset      100000;
+    }
+    \endverbatim
+
+    Where the entries comprise:
+    \table
+        Property     | Description             | Required | Default value
+        type         | Type name: reference    | yes      |
+        position     | Position to sample      | yes      |
+        field        | Name of field           | yes      |
+        reult        | Name of result field    | no       | reference(<field>)
+        scale        | Scale value             | no       | 1
+        offset       | Offset value            | no       | zero
+        result       | Name of result field    | no       | <function name>
+        log          | Log to standard output  | no       | yes
+    \endtable
+
+
+See also
+    Foam::functionObjects::fieldExpression
+    Foam::functionObjects::fvMeshFunctionObject
+
+SourceFiles
+    reference.C
+
+\*---------------------------------------------------------------------------*/
+
+#ifndef functionObjects_reference_H
+#define functionObjects_reference_H
+
+#include "fieldExpression.H"
+#include "point.H"
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+namespace Foam
+{
+namespace functionObjects
+{
+
+/*---------------------------------------------------------------------------*\
+                          Class reference Declaration
+\*---------------------------------------------------------------------------*/
+
+class reference
+:
+    public fieldExpression
+{
+    // Private data
+
+        //- Local copy of dictionary used for construction
+        dictionary localDict_;
+
+        //- Sample location
+        point position_;
+
+        //- Sample cell
+        label celli_;
+
+        //- Interpolation scheme
+        word interpolationScheme_;
+
+        //- Scale factor
+        scalar scale_;
+
+
+    // Private Member Functions
+
+        //- Calculate the reference field and return true if successful
+        virtual bool calc();
+
+        //- Utility function to calc a given type of field
+        template<class Type>
+        bool calcType();
+
+
+public:
+
+    //- Runtime type information
+    TypeName("reference");
+
+
+    // Constructors
+
+        //- Construct from Time and dictionary
+        reference
+        (
+            const word& name,
+            const Time& runTime,
+            const dictionary& dict
+        );
+
+
+    //- Destructor
+    virtual ~reference();
+
+
+    // Public Member Functions
+
+        //- Read the fieldExpression data
+        virtual bool read(const dictionary& dict);
+};
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+} // End namespace functionObjects
+} // End namespace Foam
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+#ifdef NoRepository
+    #include "referenceTemplates.C"
+#endif
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+#endif
+
+// ************************************************************************* //