[Dune] IndexSet Update

[Christian Engwer]

> I have extended the IndexSet by the Iterator and corresponding begin/end
> methods. In order to do this in the base class an additional template
> parameter was necessary which in turn required changes in the GridTraits
> again. I am very sorry, but that should have been it.

I support this proposal. And I encourage all Dune developers to say at
least "yes" or "no" or "don't care", so that we can destinguish
between "don't care" and missing answers :-)

Cheers Christian