Move _model_to_fit_params out of ParametricModel

Moved `ParametricModel._model_to_fit_params` to being a static method in `Fitter`; this is the first step in separating concerns so that only Fitters are responsible for getting their inputs into the correct format they need.  This later might even move into a separate subclass of `Fitter` since this is particular for getting parameters into the form they need to be in for use with `scipy.optimize` functions.

This allowed elimitating a lot of code rot in the process.

This did require outright removing a handful of test cases, but those tests would be better handled within the fitter tests anyways--simply marking some parameters as constrained should have no bearing on a *model's* behavior by itself.  In a later PR I will add new tests that replace these ones--that ensure that parameter constraints are handled properly by all included fitters.

This will be the first of several PRs related to further decoupling the implementation of models from how they are used by fitters.  Most of these PRs will have some dependency on each other but I will label when that is the case.  I want to still split the work up into multiple PRs so it's easier to keep track of individual changes.  Several of these changes can also be decoupled from each other if need be.