maybe switch to using mpi4py MPIPoolExecutor

[Erin Sheldon]

This removes the dependency on mpi wrappers like navy

In slurm we need this sort of more verbose call

srun -n $SLURM_NTASKS \
    --mpi=pmi2 \
    python -m mpi4py.futures $(which mdet-lsst-sim-mpi) ...args...