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.
What about converting the variable 'preconditioner' from std_cxx11::shared_ptr<Epetra_Operator> into Teuchos::RCP<Epetra_Operator> then? RCP and shared_ptr are pretty equivalent in their interface so if MueLu insists on returning an RCP we should switch if it simplifies code.