fixes #13

- Replaced Constants.h with std::numeric_limits, see also
  https://jlk.fjfi.cvut.cz/gitlab/mmg/tnl-dev/issues/3
- Made the initialValue method of reduction opeerations static and
  constexpr.

Add StaticVectorFor - for cycle doing all combinations of directions

Heavy Refactorization of DistributedGrids:
	-> Reordered Directions
	-> Add some useless Direction helper functions
	-> Set Up DistributedGrid_Base - shared base with dim template parameter
	-> Add support for communicationGroup created for DistributedGrid (MPI communicator support)

And of course our implementation is faster ;-)

Only MPICommunicator and DistributedGrid is implemented.