[!300] Implement BDMatrix::solve

Merge branch 'implement-bdmatrix-solve' into 'master'

ref:core/dune-istl This needs [!299].

See merge request [!300]

  [!299]: gitlab.dune-project.org/NoneNone/merge_requests/299
  [!300]: gitlab.dune-project.org/core/dune-istl/merge_requests/300