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/functionObject/functionObject.C

@@ -141,6 +141,12 @@ bool Foam::functionObject::read(const dictionary& dict)
 }
 
 
+bool Foam::functionObject::execute(const label)
+{
+    return true;
+}
+
+
 bool Foam::functionObject::end()
 {
     return true;