MPI Support

[zonca]

Started working on MPI support.

Having a Model.read_map method instead of a read_map function makes it easy to access self.nside, self.mpi_comm and self.pixel_indices without passing those around everywhere like in PySM 2.

Next:

• [x] consistently use the new self.read_map everywhere and make sure serial unit tests pass
• [x] implement the data distribution for the MPI case based on the TOAST PySM operator
• [ ] implement distributed smoothing with libsharp, also based on the TOAST PySM operator
• [x] implement testing for the partial map functionality
• [x] implement testing for MPI support

[zonca]

need to integrate the MPI distributions into Model, then implement smoothing

[zonca]

I'll implement smoothing with libsharp in another pull request this is good to merge.

still some trouble installing libsharp on travis, but that is not high priority, tests pass locally.