make csv output optional; return df as default

[vpchung]

Changelog

• add output=None as an arg to get_LesionWiseResults()
• add logic for creating an output CSV if output is passed a value
• calling get_LesionWiseResults() will now return the results dataframe

Nit: should eval.py also be renamed? The module name collides with Python's build-in eval() method.