model interpolation with polynomials causes ringing

Current method of model interpolation in [sdf.model.fnujy](https://github.com/drgmk/sdf/blob/c77b72e4317602f4be099634024dc661892345e2/sdf/model.py#L195) uses polynomials, and when flux is changing steeply (i.e. short side of a blackbody) ringing results.