Simplify code using asMatrix()

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).

This is a clone of !288 (closed). Is was created in order to tell the CI to use the corresponding branch in dune-common using a matching branch name.