Feature/pip installable

[apizzuto]

PR features a few things:

• Change repo structure so that everything is importable via a firesong module
• setup.py script so that users can pip install firesong with pip install /path/to/FIRESONG/
• Github Actions .yaml file for automating testing (might need to tweak this to apply to all branches once we merge)
• A small bug fix in LEGEND that was using a fixed number of bins