functionObjectList: Rationalized and simplified the handling of disabled functionObjects

Simplified and generalized the handling of functionObjects which fail to
construct by removing them from the list rather than maintaining an
"enabled" switch in each functionObject.

src/OpenFOAM/db/functionObjects/functionObject/functionObject.H

@@ -186,8 +186,9 @@ public:
         //- Called at the start of the time-loop
         virtual bool start() = 0;
 
-        //- Called at each ++ or += of the time-loop. forceWrite overrides the
-        //  outputControl behaviour.
+        //- Called at each ++ or += of the time-loop. forceWrite overrides
+        //  the usual outputControl behaviour and forces writing always
+        //  (used in post-processing mode)
         virtual bool execute(const bool forceWrite) = 0;
 
         //- Called when Time::run() determines that the time-loop exits.