Rebase (reverse) monolithic coupling on DuMuX main branch

The reversed monolithic coupling will not be merged into the main branch of DuMuX. We should merge it into this repository as we use it for testing. We might want to drop support for it in the near futures.

What does "reverse monolithic coupling mean?

• The monolithic solver couples free and porous-medium flow by enforcing a pressure boundary condition on the porous-media domain and a velocity/mass-flux boundary condition on the free flow. The "normal" monolithic solver applied the boundary/coupling conditions in the opposite way.