Simple implementation of the client-side of PMI.

This is typically provided by resource managers such as SLURM and thus not
needed.  In environments without SLURM, this allows us to use mpiexec.hydra to
launch jobs.

MPICH provides a simple implementation which can be found under:

    http://git.mpich.org/mpich.git/tree/HEAD:/src/pmi/simple

This code was directly inspired by Sandia OpenSHMEM's use of it:

    https://github.com/Sandia-OpenSHMEM/SOS/tree/master/pmi-simple

Files copied from Sandia OpenSHMEM:

    - mpl.h

See LICENSE-SandiaOpenSHMEM in parent directory.

Files copied from MPICH:

    - pmi.h
    - simple_pmi.c
    - simple_pmiutil.c
    - simple_pmiutil.h

See LICENSE-MPICH in this directory.
