Update grmSimulation.py

The goals of the final exam are to (1) calculate and export
conventional treatment effects and (2) make simulation parallel via
MPI. I achieve both by modifying only grmSimulation. In particular, I
treat grmSimulation.simulate() as the master simulation that combines
data and treatments from parallel worker simulations, which are
computed on each of the parallel processes by
grmSimulation.workerSimulate().