[draft] FEAT: add effective spins

Based entirely off of Isobel's fork, see her MR here: https://git.ligo.org/isobel.romero-shaw/mock-PE/-/merge_requests/1

A few changes to that fork:

• remove bilby dependence
• create a flag that lets you include chi_eff vs not. (Not sure if this is actually necessary but we can try it)
• integrate spin prior into transforms.py functions
• create only one example config file

I'm also using this as an opportunity to clean up the code a bit, removing some functions and packages that were never really used.

Closes #1