Note that it subclasses `Model`, not `ParametricModel`. Subclasses of `Model` are not to be fitted.
But that made me look at fitting.py and oops, there's no check for that. I'll add it to the `Fitter` class.
And should probably add an example of a non-fittable model to new.rst.  Is that sufficient?
Or did you mean adding a `ModelModifier` class as a mixin? (That maybe an unnecessary complication.)