Using the new Impl::asVector() from dune-common we can get rid of
several Hybrid::isElse() constructions used to implement special cases for isNumber,
undefined behaviour invoked by toVector() due to the latter violating strict aliasing rules.
This also removed the dangerous toVector() methods. This is only a partial solution to #63 (closed). We also need to do the same for toMatrix() which requires an analogue Impl::asMatrix() function and a ScalarMatrixView wrapper class.
