Add flowMC to the sampling options for light curve fits

[kdesoto-astro]

Adds flowMC as an alternative to dynesty, SVI, and NUTS in the sampling suite. Will be roughly broken into the following steps:

• Setting up function names, docstrings, and expected outputs for each function
• Implementing the method that samples from the posterior space
• Add likelihood calculations + fitting optimization routines
• Ensuring compatibility with existing unit tests, adding package dependencies to existing list
• Adding new unit tests for flowMC.

This is expected to be completed by Friday, 7/21/23.