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/fieldsExpression/fieldsExpression.C

@@ -123,6 +123,12 @@ bool Foam::functionObjects::fieldsExpression::read(const dictionary& dict)
 }
 
 
+Foam::wordList Foam::functionObjects::fieldsExpression::fields() const
+{
+    return fieldNames_;
+}
+
+
 bool Foam::functionObjects::fieldsExpression::execute()
 {
     if (!calc())