SubGridIntersection<GridImp> should allow access to hostIntersection

In the same way that SubGridEntity<GridImp> supplies a hostEntity function to get access to the corresponding entity in the host grid, SubGridIntersection<GridImp>should provide this too.

It is already possible to get the host grid intersection indirectly by going through the entity and using the index of the intersection, but this is tedious and reduces readability.

The Hostgrid intersection is also already stored in the SubGridIntersection (as insideIntersect_), so implementation of this is trivial (and supplied with this pull request).

Unless there is a higher level design reason why this should not be possible, please allow this merge.