[grid.provider.cube] update const grid_ptr() variant
[grid.provider.cube] update const grid_ptr() variant

To be on the safe side, when someone does something like

    const auto& grid = *grid_provider.grid_ptr();

which should be valid, since the underlying shared_ptr is guaranteed (by
the grid provider) to not change.

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