Merge branch '55-amg-set-hasDiagonal-to-true' into 'master'

ref:core/dune-istl Thanks to Nils-Arne Dreier for noticing.

Closes: [#55]

See merge request [core/dune-istl!236]

  [#55]: gitlab.dune-project.org/NoneNone/issues/55
  [core/dune-istl!236]: gitlab.dune-project.org/core/dune-istl/merge_requests/236


Closes #55