[!375] Feature/blocklevel detection

Merge branch 'feature/blocklevel-detection' into 'master'

ref:core/dune-istl\> Proposal for extending [!168] to MultiTypeBlockMatrix. As
the blockLevel for MultiTypeBlockMatrix is sometimes not uniquely determined
this proposed three functions minBlockLevel, maxBlockLevel and blockLevel. The
first two always work. The last one is only enabled if the first two return
equal levels. There is an additional function
`constexpr bool hasUniqueBlockLevel()` which checks this condition at compile
time.

Depends on the fix in hybrid utilities [dune-common!794]

See merge request [core/dune-istl!375]

  [!168]: gitlab.dune-project.org/NoneNone/merge_requests/168
  [dune-common!794]: gitlab.dune-project.org/Nonedune-common/merge_requests/794
  [core/dune-istl!375]: gitlab.dune-project.org/core/dune-istl/merge_requests/375