[grid.provider.interface] refactor to allow for const grids, fixes #22
[grid.provider.interface] refactor to allow for const grids, fixes #22

Grid view may be constructed from a const grid, grid parts not. Thus the
ConstProviderInterface provides only views and the ProviderInterface also
provides parts.

renemilk/DUNE-Stuff,renemilk/DUNE-Stuff,renemilk/DUNE-Stuff