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.
Is there any difference from the ML code here? Are the MueLu parameter lists really that similar or how does it work with ML parameters? I realize that this should be replaced by the code below at some point, though.