ENH: added functionObject::execute(int) method

- this is a provision for defining execute actions that can be called
  largely independently of the normal time-loop constraints. This can
  be useful to provide hooks for sub-cycling, or to define an action
  that can be triggered manually or on some other event.

src/OpenFOAM/db/functionObjects/timeControl/timeControlFunctionObject.H

@@ -206,6 +206,9 @@ public:
             //  forces execution (used in post-processing mode)
             virtual bool execute();
 
+            //- Execute using the specified subIndex.
+            virtual bool execute(const label subIndex);
+
             //- Called at each ++ or += of the time-loop.
             //  postProcess overrides the usual writeControl behaviour and
             //  forces writing (used in post-processing mode)