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.
Alternatively, maybe we should just not drive MueLu via the ML interface. MueLu is our future anyway, so we could decide to not even try to be compatible. (I suppose this would mean to use the less comfortable interface you described, though.)