ENH: Updated fieldAverage FO windowing. See #525

When specifying the averaging data, a new `windowType` option is
available, taking the values:

- none: no windowing
- approximate: past functionality (v1706 and earlier)
- exact: exact moving average - will store and write (for restart) all
  fields in the window

src/OpenFOAM/db/functionObjects/functionObjectList/functionObjectList.C

@@ -579,9 +579,8 @@ bool Foam::functionObjectList::execute()
             }
         }
     }
-
     // Force writing of state dictionary after function object execution
-    if (time_.outputTime())
+    if (time_.writeTime())
     {
         label oldPrecision = IOstream::precision_;
         IOstream::precision_ = 16;