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/OpenFOAM/db/functionObjects/timeControl/timeControlFunctionObject.C

@@ -95,6 +95,12 @@ Foam::functionObjects::timeControl::timeControl
 
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
+Foam::wordList Foam::functionObjects::timeControl::fields() const
+{
+    return foPtr_->fields();
+}
+
+
 bool Foam::functionObjects::timeControl::executeAtStart() const
 {
     return foPtr_->executeAtStart();