MPI disable option

[msyriac]

On some systems (e.g NERSC login node where one might want to do some quick debugging), enlib's attempts to load MPI result in a fatal error that cannot be caught:

In [1]: from enlib import mpi
[Mon Apr  2 14:30:13 2018] [unknown] Fatal error in PMPI_Init_thread: Other MPI error, error stack:
MPIR_Init_thread(537): 
MPID_Init(247).......: channel initialization failed
MPID_Init(636).......:  PMI2 init failed: 1 
Aborted (core dumped)

This PR recognizes the environment variable DISABLE_MPI , and simply avoids loading mpi4py if that environment variable is set to true.