@adonath I was hoping to not have any models related imports in the fitting module and keep models and fitting completely separate. One reason is that in the future this may introduce more model specific dependencies. So would it make more sense to make `col_deriv` an attribute of `ParametricModel` with a default value of 1 and set to 0 in `PolynomialModel` and `OrthogPolyBase`? Then a fitter would use `model.col_deriv`.