[draft][bugfix] Don't use mv and mtv with raw random access containers

The Dune matrix types do not support MatrixType::mv(x,y) passing plain random access containers as vectors but require x.N() and y.N() if bound checks are enabled. As a fix we introduce an internal Impl::DensseVectorView class that provides the DenseVector interface for generic random access containers.

Notice that this class is not tested for general use and should be replaced by DenseVectorSpan once this was added to dune-common and is sufficiently generic for beeing used here.