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.

etc/codeTemplates/dynamicCode/codedFunctionObjectTemplate.H

@@ -96,6 +96,9 @@ public:
         //- Read the system calls
         virtual bool read(const dictionary&);
 
+        //- Return the list of fields required
+        virtual wordList fields() const;
+
         //- Execute the "executeCalls" at each time-step
         virtual bool execute();