I don't think I agree with moving this into the model class.  Its purpose is very decidedly tied to the implementation of the fitters, in that it's an adapter from the `deriv` methods on models to the interface expected by the scipy fitters.  This could even just be a stand-alone function in the fitters module that acts as a decorator--given a model it spits out a function that curries the model with a new wrapper function for its `deriv` method.