Restructure linearsolver parallel
- Deduce vector and matrix type from solve
- Actually use sequential solver if we compute on one process (before a "parallel" solver was used)
- Fix "sequential" constructor of the AMG backend
- Rename to AMGBiCGSTABBackend
- LinearSolverTraits only need GridGeometry now
- enable box with overlap (big thanks to @blattms for implementing the draft for this (see !1861 (closed)) and giving the necessary hint on how to do this)
- restructure the LinearSolverTraits
I consider the internal structure of the AMG non-public so we don't have to deprecate things. Opinions?
This is suggestion to fix #805 (closed).