Enable MPI

[sbacchio]

MPI at the moment is disabled.

Solving this issue requires:

• Add mpi in setup.py and CMakelist.txt (done but commented)
• Replace MPI_COMM_WOLRD with a g_comm everywhere in tmLQCD (patch)
• Enable halos in su3 fields
• Check what variables needs to be set for making it work
• Add tests
• Add workflow for testing both with and without MPI