Clearer names for saving and running

Initially I referred to many things via EM such as EM_trial_simulation,
but this stopped making sense when we also incorporated the WM side.
Also, I had hardcoded the place to save (or load) simulation results
which doesn't generalize to other people running my script cause they
would have to change this value in many places.  Lastly, most of my
simulation result variables were called this.

Add central save location in PM_task via PM_task.SAVE_DIRECTORY
property.  Modify save and loading simulation functions to use more
appropriate variable names as well as new PM_task.SAVE_DIRECTORY
property.