DAGP for slab grids

[totork]

The part that calculates the dagp variables seems to have problems with the z-periodicity in a slab grid.

It would be nice to fix that so i can use them in MMS tests.

@dschwoerer

[dschwoerer]

Is it not easier for you to just fixup that grid?
For a slab you know that the values in the bulk should also be used for the boundaries. I guess it is a bit more tricky to do that for DAGP in general, as DAGP uses lab coordinates, but they do not exist.

If you come up with a nice interface, I am happy to implement in DAGP, otherwise I think it might be easier as a post-processing step ...