unfortunately, that does not work either:

```
/home/and/src/dune-cornerpoint/dune/grid/cpgrid/CpGridData.hpp:739:9: error: static_cast from 'Dune::cpgrid::GlobalIdSet *' to 'Dune::cpgrid::GlobalIdMapping *' is not allowed
```

Fortunately, I've found the cause for this: The `Indexsets.hpp` header has not been included up to that point and `static_casts` of undefined classes cannot work. I got to say that clang's error message here is _very_ confusing...

I've updated the PR accordingly, so I as far as I'm aware it's now ready for The Green Button.