Added a resetOnOutput switch to fieldAverage to allow the averaging to be only of the preceding writeInterval for transient cases.

- change system/controlDict to use functions {..} instead of functions (..);
  * This is internally more efficient
- fixed formatting of system/controlDict functions entry

- pedantic change: use 'return 0' instead of 'return(0)' in the applications,
  since return is a C/C++ keyword, not a function.

- track previous time index and to avoid recalculating averages.
- make sure averages are up-to-date before calling write

- added end() method to functionObject, functionObjectList & associated classes
- moved outputFilters from src/sampling -> src/OpenFOAM/db/functionObjects