GeometricField, volFields: Added experimental member function ".v()" and perfix operator "~"

both of which return the dimensionedInternalField for volFields only.

These will be useful in FV equation source term expressions which need
not evaluate boundary conditions.

applications/test/volField/Test-volField.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011-2015 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2016 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -52,6 +52,10 @@ int main(int argc, char *argv[])
         mesh
     );
 
+    Info<< nl
+        << "p.v().size(): "
+        << p.v().size() << endl;
+
     Info<< "Reading field U\n" << endl;
     volVectorField U
     (