[WIP] Simplify code using asMatrix()

Carsten Gräserrequested to merge
feature/use-scalarmatrixview into master

This is based on dune-common!656 (merged) which introduces Impl::asMatrix() using this we can avoid many Hybrid::ifElse constructions which increase complexity of the implementation and error messages. Furthermore we can drop toMatrix() which fixed #63 (closed).

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