[Dune] dune-grid does not compile when implementations of EntityPointer and Iterator are not interoperable

[Christian Engwer]

> 1) Completely remove the cast (perhaps with an optional configure flag to keep it one more round).
> 2) Change the implementation to work correctly for non-interoperable types. This would make it possible for grid
>   implementors to selectively disable the cast for their grids, which might simplify implementations (see e.g. your
>   comment about the required memory layout of EntityPointer) and possibly save users from a bunch of ugly
>   problems, while still staying backward compatible.
> 3) Leave everything as it is now.

I vote for 1)

Christian