Add support for AMG improvements when using inofficial 2.2 DUNE release with CMake support

In the inofficial 2.2 DUNE release with CMake support I have backported some new ISTL features from the trunk. Namely, a faster hierarchy setup of the AMG and a new AMG variant that better utilizes the available memory bandwidth for a faster solve phase.

These commits make the new fast AMG version available as an additional solver. If it is not available, we simply fall-back to the standard AMG.

I will leave it to the release managers, whether to incorporate this into the release or not.

Comments are very welcome.