Do not force grid to use Dune::Entity.
This is test code and should be as generic as it can be. Previously (but not since ever) it assumed that Grid::Entity is always a subclass of Dune::Entity. This seems to hard a restriction as it is only used to check that the grid has a hasEntity<codim> capability for all its Entities (which is a bug IMHO, too). Therefore this commit makes CheckSubEntityGeometry a template struct that has the Grid as a parameter and a nested Operation struct with template parameter codim that is used in the ForLoop for testing.