What this MR does / why does DuMux need it:
It allows using the box method with overlapping grids and nonzero overlapSize (the prerequisite, previously). In the case of zero overlap the old approach (nonoverlapping linear operators, preconditioners and scalarproducts) is used. Otherwise we switch to using the overlapping counterparts.
Special notes for your reviewer:
- As far as I can see failed tests stay the same with this when compared to master.
- Might need some further testing
- Rebased onto a version of master from Friday (that was quite a huge effort since the backend files did change quite a lot).