[Dune] EntityPointer finished

[Christian Engwer]

Hi,

the EntityPointer is finished and works fine. It only needs very small
changes in the grid implementation, all tricks are done in the wrapper
class.

I already have sgrid working under the new api. And! I must have
repaired the slow-iterator-constructor-bug ... sgrid it nearly 10
times faster than before :-)

I would write a small overview of the changes and commit the new
headers. Ay objections?

Cheers Christian

PS: These changes are _really_ simple ... not comparable to the const changes.