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.
That's correct. I was just wondering what happens if we compile deal.II in 64-bit mode. Then we create 64-bit Epetra matrices. The code should probably have something like
  Assert (sizeof(global_dof_index) == sizeof(int), ExcMessage ("...can't do this..."));
just to make sure. Unless we already get an intelligible error message to the same effect from Trilinos.