Split grid manager specialization into different headers

Problem: When updating the code of a grid manager, all tests have to be recompiled, not only the tests that use this particular grid manager. Example: I pull an upstream bugfix in dune-foamgrid, everything has to be compiled again, not only the tests using foamgrid.

Possible solution: Each specialization of Dumux::GridManager could go in its own header (which includes the base definition and header for this particular grid manager)

• Disadvantage: You would need to change your includes if you use a different grid manager
• Advantage: Only the code for grids that are actually used has to be included. Update of a grid manager only affects those applications that really use this grid manager