[Dune] YaspGrid, periodicity and boundary segment indices

[Christian Engwer]

Hi Martin,

> clear whether YaspGrid works properly in a periodic setting (I think the
> grid check still fails).

As far as I see it, the grid check is too strict. It doesn't fail fo
other grids, because they arent tested as seq. periodic grids.

The problem is that we loose a lot for the non-periodic case, if use a
less restrictive test.

Christian