Fix sim input filename keyword in make_sim_inputs.py

This changes the sim input filename keyword (originally ``outfile``) in make_sim_inputs.py to match the corresponding keyword as defined in ``__init__.py``.

I originally expected the issue here to reside in ``__init__.py``, however after further study on how the ``argparse`` and ``inspect`` modules worked, I found that a quick change to a keyword name in ``make_sim_inputs.py`` fixed the problem. This is why I named the branch "fix_init".