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.C

@@ -0,0 +1,133 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  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/>.
+
+\*---------------------------------------------------------------------------*/
+
+#include "reference.H"
+#include "addToRunTimeSelectionTable.H"
+
+// * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
+
+namespace Foam
+{
+namespace functionObjects
+{
+    defineTypeNameAndDebug(reference, 0);
+
+    addToRunTimeSelectionTable
+    (
+        functionObject,
+        reference,
+        dictionary
+    );
+}
+}
+
+
+// * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
+
+bool Foam::functionObjects::reference::calc()
+{
+    Log << type() << " " << name() << " output:" << nl;
+
+    bool processed = calcType<scalar>();
+    processed = processed || calcType<vector>();
+    processed = processed || calcType<sphericalTensor>();
+    processed = processed || calcType<symmTensor>();
+    processed = processed || calcType<tensor>();
+
+    Log << endl;
+
+
+    return returnReduce(processed, orOp<bool>());
+}
+
+
+// * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
+
+Foam::functionObjects::reference::reference
+(
+    const word& name,
+    const Time& runTime,
+    const dictionary& dict
+)
+:
+    fieldExpression(name, runTime, dict),
+    localDict_(dict),
+    position_(point::zero),
+    celli_(-1),
+    interpolationScheme_("cell"),
+    scale_(1)
+{
+    read(dict);
+
+    // Forcing default result name to include the name of the field
+    setResultName(typeName, word::null);
+}
+
+
+// * * * * * * * * * * * * * * * * Destructor  * * * * * * * * * * * * * * * //
+
+Foam::functionObjects::reference::~reference()
+{}
+
+
+bool Foam::functionObjects::reference::read(const dictionary& dict)
+{
+    if (fieldExpression::read(dict))
+    {
+        localDict_ = dict;
+
+        dict.lookup("position") >> position_;
+
+        interpolationScheme_ =
+            dict.lookupOrDefault<word>("interpolationScheme", "cell");
+
+
+        dict.readIfPresent("scale", scale_);
+
+        celli_ = mesh_.findCell(position_);
+
+        label celli = returnReduce(celli_, maxOp<label>());
+
+        if (celli == -1)
+        {
+            FatalErrorInFunction
+                << "Sample cell could not be found at position " << position_
+                << exit(FatalError);
+        }
+
+        Log << type() << " " << name() << nl
+            << "    field: " << fieldName_ << nl
+            << "    sample position: " << position_ << nl
+            << "    scale: " << scale_ << nl
+            << endl;
+
+        return true;
+    }
+
+    return false;
+}
+
+
+// ************************************************************************* //