Enable fitting ridge lines to non-linear functions

While scaling relationships have typically been fit with linear functions, these may not be the best descriptors of the relationships. In the case of the resolved SFMS, this is certainly the case for the high-mass end of the distribution.

Functions themselves and wrapper functions can be added in fit_funcs.py. Those options will need to be enabled within the Classes that manage data (GalDat.py). Ideally, where the classes call the functions to fit could be generalized.