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).