Oliver Sander authored Nov 23, 2018 and Timo Koch committed Nov 23, 2018

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]

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

(cherry picked from commit 54ee6051)