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.
Well, these numbers suggest that the preconditioner is not effective like if there was no multigrid hierarchy. We double the number of DoFs (this is a 2D problem) and get twice the iteration count. Bruno, could you please check what happens? I.e., also including some MueLu output to see if it indicates a problem.