Fitter without model ref

Here are my recommended tweaks to this PR.

It keeps `_model_to_fit_params` as a `ParametricModel` method, but puts the code for `_fit_params_to_model` back in Fitter so at least there's some two-way adaptation. I moved `_wrap_deriv` and `_deriv_with_constraints` into `NonLinearLSQFitter` and `LinearLSQFitter` respectively, as they are very decidedly implementation details of those fitters alone. I left weights alone for now.

This is also rebased on master so it may not apply cleanly until your rebased version of this branch is pushed to GH.