Feature/multitype arithmetic

I added some code to deal with matrices of type MultiTypeBlockMatrix. I am not quite convinced that the way in which I extended the traits are the best way to go. In particular, the isStatic is ambiguous, because actually all other matrices but BCRS are static as well, but do not work well with my implementation. Therefore, I am happy about any kind of ideas for improvements.

The technology is there. I found a useful code basis in dune-solvers and I believe it is reasonable to migrate algorithm.hh to dune-matrix-vector. Only minor parts from dune-solvers arithmetic.hh are needed. The vast hybrid utility power is in dune-common now. All static/dynamic & sparse/dense-magic is handled therein.