Make NERSC/MPI issue more transparent

[msyriac]

import mpi4py fails on NERSC login nodes. To get around this, pixell respects the DISABLE_MPI=true environment variable as implemented in pixell/mpi.py. But a typical user wont know they need to do this on NERSC. What do we do about that?

Because of this issue, running py.test for example seems to just Abort without an informative message. (The problem is only revealed with "py.test -s")