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.
My comment was mostly that I didn't understand what the text in the //comment was saying. I didn't want to imply that you should change anything in the code, but to explain the issue better/differently in the //comment.