When you get to separating optimizers from statistics, it becomes necessary for `fitter_to_model_params` to be a function. You may consider making `model_to_fit_params` a function as well.

(The reason is that `LinearLSQFitter` and `NonLinearLSQFitter` (called `LevMarLSQFitter` in #1914) can't be split into an optimizer and a statistic and so don't inherit from `Fitter`.)