Improve ParallelBoundary code
Builds on #2629
Allows for handling regions of short connection length differently from region of long connection length
This allows to use higher order stencil in general, without breaking for the points that don't support this.
Requires #2640
This looks massive, but I guess it's because it includes next or something into the base branch? @dschwoerer Please could you update the base branch so the changes here are easier to see?
My bad, I had forgotten this was a PR against par-bc-cleanup, not next. Thus merging next did only make things worse. Now I merged in par-bc-cleanup, and it is down to 16 commits.