[!241] MultiTypeBlockVector: Inherit tuple constructors

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 core/dune-istl!241

(cherry picked from commit 54ee6051)

@oliver.sander Does this have a chance of getting backported? I have a used case in dumux for the upcoming release where the code would be much more verbose than without this change, and we would also need to introduce an interface that we would change again with dune 2.7.

I don't object to this getting merged, but I am not 2.6 release manager. For that reason I don't fill entitled to simply press 'merge'.

@martin.nolte @andreas.dedner

merged

mentioned in commit bb0fee97