[Dune] GhostEntity

[Christian Engwer]

Dear all,

we (in Stuttgart) were wandering what the properties of ghostentities
are. It is clear that ghostentities may not implement all features of
a full blown entity. In the grid paper and in the doxygen
documentation, I could not find a clear definition of the guaranteed
features of ghostentities.

We need to agree on a mandatory feature set of ghostentities.

Cheers Christian