functionObjects: Added fields() function to provide list of required fields to postProcess

With this change each functionObject provides the list of fields required so
that the postProcess utility can pre-load them before executing the list of
functionObjects.  This provides a more convenient interface than using the
-field or -fields command-line options to postProcess which are now redundant.

src/functionObjects/field/fieldValues/fieldValue/fieldValue.H

@@ -163,8 +163,8 @@ public:
         //- Return the region name
         inline const word& regionName() const;
 
-        //- Return the list of field names
-        inline const wordList& fields() const;
+        //- Return the list of fields required
+        inline virtual wordList fields() const;
 
         //- Return the output field values flag
         inline const Switch& writeFields() const;

src/functionObjects/field/fieldValues/fieldValue/fieldValueI.H

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     | Website:  https://openfoam.org
-    \\  /    A nd           | Copyright (C) 2011-2018 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2021 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -40,7 +40,7 @@ inline const Foam::word& Foam::functionObjects::fieldValue::regionName() const
 }
 
 
-inline const Foam::wordList& Foam::functionObjects::fieldValue::fields() const
+inline Foam::wordList Foam::functionObjects::fieldValue::fields() const
 {
     return fields_;
 }