[!241] MultiTypeBlockVector: Inherit tuple constructors

Timo Kochrequested to merge
tkoch/dune-istl:backport/feature/mtbv-inherit-tuple-constructors into releases/2.6

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)

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