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 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.
Merge pull request #2264 from embray/decouple-fitters

Move _model_to_fit_params out of ParametricModel