[!242] [!241] MultiTypeBlockVector: Inherit tuple constructors

Merge branch 'backport/feature/mtbv-inherit-tuple-constructors' into 'releases/2.6'

ref:core/dune-istl Merge branch 'feature/mtbv-inherit-tuple-constructors' into
'master'

ref:core/dune-istl

-   Makes member function count const
-   Inherits tuple constructors so that MultiTypeBlockVector can be
    constructed from existing Vectors, or we can even construct it with
    references to existing vectors.
-   Test construction from references in the test
-   Fixes the static_assert in the test that shouldn't really have compiled
    before

See merge request [[!241]]

(cherry picked from commit 54ee6051)

See merge request [!242]

  [!241]: gitlab.dune-project.org/core/dune-istl/merge_requests/241
  [!242]: gitlab.dune-project.org/core/dune-istl/merge_requests/242