[Dune] Periodic boundary conditions in YaspGrid

[Christian Engwer]

> in the YaspGrid constructor, directions can be labeled as periodic. 
> However, I don't see a difference in the result. Shouldn't vertices at 
> opposite faces in that direction just be assigned the same global node 
> number? Apparently, that doesn't happen, so how do you find out which node 
> is a neighbor then?

You are right, "front" nodes on a periodic boundary should be assigned
the same globalid, although they have different local indices.

If this does not work, it is a bug. Could you provide a minimal
regression test and create a bug report?

Christian