[Dune] higher codim entities.

[Christian Engwer]

Hi,

> If the index sets are already working then it is no problem to implement
> the entities, because the geometies can be easily implemented with use
> of the geometry of codim 0 and the already existing reference elements.
> All other method of the higher codim entities are trival to implement.
> 
> Then, if the entity method works for all codims, the Level and Leaf
> iterators can also be implemented using the entity method of the ocdim 0
> entities.

This is a feasible way, I would supprt this proposal. Although I would
suggest to concentrate on the parallel grid interface and the 1.0
release. The additional entities are of low priority.

CU Christian