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.
I tried to set the the solver myself but it doesn't work. The bug is easy to fix https://software.sandia.gov/bugzilla/show_bug.cgi?id=6256 but I guess they didn't have time yet. We could use the new interface but the interface will probably be different if we use Tpetra or Epetra because Ifpack only supports Epetra and Ifpack2 only supports Tpetra...