WIP: Polaron dispersion functionality

[Chiffafox]

This pull request (re-)introduces polaron tracking functionality into the codebase. It is available for all Solver types, at both zero and finite T. Tests are also included. The "scipy fit" method is used for the fit.

Also added are the processing of dispersion function results, and an effective mass fit (untested).

Have not yet tested "black" and other simple checks and still missing some tests, and no documentation yet -- hence the WIP tag.

[matthewcarbone]

@Chiffafox this is really great, thank you. My two main concerns are:

• Checks are failing 😁
• There seems to be a ton of repetition between base.py and solvers.py. I didn't do a diff check to see just how much but I can't imagine we can't somehow golf this down a bit.