[!241] MultiTypeBlockVector: Inherit tuple constructors
Merge branch 'feature/mtbv-inherit-tuple-constructors' into 'master'
- 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 core/dune-istl!241
(cherry picked from commit 54ee6051)