Have a more general way to recognize models for fitting that have positional parameters #304
Description
From #210 - see #210 (comment) for more detail.
The basic gist: models like polynomials can't be "copied" trivially because you need a way to know how many parameters they have. inspect gives everything needed to recognize this, and I think I solved it in the linked comment... but it needs a PR with proper testing and everything, as it's pretty complex.