better documentation on PySEP argument options

[bch0w]

Related to #118, some argument inputs of PySEP like write_files and plot_files are not covered in the documentation, and only really visible in the direct docstrings of the source code. Since it's not expected that every User will be looking at source code doc strings, we should make some more transparent documentation that exposes the expected API of these less-than-intuitive parameters.