functionObjects::add: Add list of fields

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

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

Also refactored functionObjects::fieldsExpression to avoid code
duplication between the 'add' and 'subtract' functionObjects.

src/functionObjects/field/fieldsExpression/fieldsExpression.C

@@ -52,12 +52,12 @@ void Foam::functionObjects::fieldsExpression::setResultName
 
     if (resultName_.empty())
     {
-        if (fieldNames_ != defaultArgs)
+        if (!fieldNames_.empty())
         {
-            resultName_ = typeName + '(';
-            forAll(fieldNames_, i)
+            resultName_ = typeName + '(' + fieldNames_[0];
+            for (label i=1; i<fieldNames_.size(); i++)
             {
-                resultName_ += fieldNames_[i];
+                resultName_ += ',' + fieldNames_[i];
             }
             resultName_ += ')';
         }
@@ -85,6 +85,15 @@ Foam::functionObjects::fieldsExpression::fieldsExpression
     resultName_(resultName)
 {
     read(dict);
+
+    if (fieldNames_.size() < 2)
+    {
+        FatalIOErrorInFunction(dict)
+            << "functionObject::" << type() << " " << name
+            << " requires at least 2 fields only "
+            << fieldNames_.size() << " provided: " << fieldNames_
+            << exit(FatalIOError);
+    }
 }
 
 

src/functionObjects/field/fieldsExpression/fieldsExpression.H

@@ -71,9 +71,27 @@ protected:
 
     // Protected member functions
 
-        virtual bool calc() = 0;
+        void setResultName
+        (
+            const word& typeName,
+            const wordList& defaultArg = wordList::null()
+        );
 
-        void setResultName(const word& typeName, const wordList& defaultArg);
+        //- Call 'calcFieldType' for the given functionObject
+        //  for 'volField' and 'surfaceField' field types
+        template<class Type, class FOType>
+        bool calcFieldTypes(FOType& fo);
+
+        //- Call 'calcFieldTypes' for the given 'Type' and functionObject
+        template<class Type, class FOType>
+        bool calcType(FOType& fo);
+
+        //- Call 'calcType' for the given functionObject
+        //  for each primitive type
+        template<class FOType>
+        bool calcAllTypes(FOType& fo);
+
+        virtual bool calc() = 0;
 
 
 private:
@@ -133,6 +151,12 @@ public:
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
+#ifdef NoRepository
+    #include "fieldsExpressionTemplates.C"
+#endif
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
 #endif
 
 // ************************************************************************* //

src/functionObjects/field/fieldsExpression/fieldsExpressionTemplates.C

@@ -0,0 +1,82 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  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 "volFields.H"
+#include "surfaceFields.H"
+
+// * * * * * * * * * * * * Protected Member Functions  * * * * * * * * * * * //
+
+template<class Type, class FOType>
+bool Foam::functionObjects::fieldsExpression::calcFieldTypes(FOType& fo)
+{
+    typedef GeometricField<Type, fvPatchField, volMesh> VolFieldType;
+    typedef GeometricField<Type, fvsPatchField, surfaceMesh> SurfaceFieldType;
+
+    if (foundObject<VolFieldType>(fieldNames_[0]))
+    {
+        return store
+        (
+            resultName_,
+            fo.template calcFieldType<VolFieldType>()
+        );
+    }
+    else if (foundObject<SurfaceFieldType>(fieldNames_[0]))
+    {
+        return store
+        (
+            resultName_,
+            fo.template calcFieldType<SurfaceFieldType>()
+        );
+    }
+    else
+    {
+        return false;
+    }
+}
+
+
+template<class Type, class FOType>
+bool Foam::functionObjects::fieldsExpression::calcType(FOType& fo)
+{
+    return calcFieldTypes<Type>(fo);
+}
+
+
+template<class FOType>
+bool Foam::functionObjects::fieldsExpression::calcAllTypes(FOType& fo)
+{
+    bool processed = false;
+
+    processed = processed || fo.template calcType<scalar>(fo);
+    processed = processed || fo.template calcType<vector>(fo);
+    processed = processed || fo.template calcType<sphericalTensor>(fo);
+    processed = processed || fo.template calcType<symmTensor>(fo);
+    processed = processed || fo.template calcType<tensor>(fo);
+
+    return processed;
+}
+
+
+// ************************************************************************* //