Muelu

This pull request adds MueLu as a preconditioner. The pull request is not ready to be merged, I'm waiting for upstream to fix a bug. For now the code works exactly in the same way as ML, except for the parameter high_order_function which does not exist anymore. There is also a lot of code which is commented that can be used when we want to break compatibility with ML.
Aw, bummer -- with ML we need 19 and 28 iterations. Will this be better with a patched MueLu?