ENH: functionObjects: call execute on last time step

- old convention was that on last time step it would only call end()
and not execute()
- however this meant that e.g. the functionObjectProperties file
did not get written
- and almost all functionObjects were doing an execute() inside of end()
- new convention: call execute() on last time step, just before doing end()

src/postProcessing/functionObjects/fvTools/calcFvcGrad/calcFvcGrad.C

@@ -119,10 +119,7 @@ void Foam::calcFvcGrad::execute()
 
 void Foam::calcFvcGrad::end()
 {
-    if (active_)
-    {
-        execute();
-    }
+    // Do nothing
 }