Parallelization in mc_md.py

[bctapia]

The current method of passing np to mpiexec through a dictionary entry (e.g. 'np': 40) into md_props does actually pass the value into sim.run() as required. Thus, the code always runs in serial. Current quick fix is editing source code to sim.run(np=40, prefix='mpiexec') but this is not flexible