functionObjects::subtract: From the first field subtract the remaining fields in the list

The operation can be applied to any volume or surface fields generating a
    volume or surface scalar field.

    Example of function object specification:
    \verbatim
    Tdiff
    {
        type            subtract;
        libs            ("libfieldFunctionObjects.so");
        fields          (T Tmean);
        result          Tdiff;
        executeControl  writeTime;
        writeControl    writeTime;
    }
    \endverbatim

src/functionObjects/field/fieldsExpression/fieldsExpression.C

@@ -0,0 +1,149 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2016 OpenFOAM Foundation
+     \\/     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 "fieldsExpression.H"
+#include "dictionary.H"
+
+// * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
+
+namespace Foam
+{
+namespace functionObjects
+{
+    defineTypeNameAndDebug(fieldsExpression, 0);
+}
+}
+
+
+// * * * * * * * * * * * * * Protected Member Functions  * * * * * * * * * * //
+
+void Foam::functionObjects::fieldsExpression::setResultName
+(
+    const word& typeName,
+    const wordList& defaultArgs
+)
+{
+    if (fieldNames_.empty())
+    {
+        fieldNames_ = defaultArgs;
+    }
+
+    if (resultName_.empty())
+    {
+        if (fieldNames_ != defaultArgs)
+        {
+            resultName_ = typeName + '(';
+            forAll(fieldNames_, i)
+            {
+                resultName_ += fieldNames_[i];
+            }
+            resultName_ += ')';
+        }
+        else
+        {
+            resultName_ = typeName;
+        }
+    }
+}
+
+
+// * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
+
+Foam::functionObjects::fieldsExpression::fieldsExpression
+(
+    const word& name,
+    const Time& runTime,
+    const dictionary& dict,
+    const wordList& fieldNames,
+    const word& resultName
+)
+:
+    fvMeshFunctionObject(name, runTime, dict),
+    fieldNames_(fieldNames),
+    resultName_(resultName)
+{
+    read(dict);
+}
+
+
+// * * * * * * * * * * * * * * * * Destructor  * * * * * * * * * * * * * * * //
+
+Foam::functionObjects::fieldsExpression::~fieldsExpression()
+{}
+
+
+// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
+
+bool Foam::functionObjects::fieldsExpression::read(const dictionary& dict)
+{
+    fvMeshFunctionObject::read(dict);
+
+    if (fieldNames_.empty() || dict.found("fields"))
+    {
+        dict.lookup("fields") >> fieldNames_;
+    }
+
+    if (dict.found("result"))
+    {
+        dict.lookup("result") >> resultName_;
+    }
+
+    return true;
+}
+
+
+bool Foam::functionObjects::fieldsExpression::execute()
+{
+    if (!calc())
+    {
+        Warning
+            << "    functionObjects::" << type() << " " << name()
+            << " cannot find required fields " << fieldNames_ << endl;
+
+        // Clear the result fields from the objectRegistry if present
+        clear();
+
+        return false;
+    }
+    else
+    {
+        return true;
+    }
+}
+
+
+bool Foam::functionObjects::fieldsExpression::write()
+{
+    return writeObject(resultName_);
+}
+
+
+bool Foam::functionObjects::fieldsExpression::clear()
+{
+    return clearObject(resultName_);
+}
+
+
+// ************************************************************************* //