fixes #18

The method was either returning true or throwing an exception, so it can
simply return void or throw an exception.

Fixed handling of the boundary conditions for explicit solver - introduced a method Problem::applyBoundaryConditions.

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)