Save polynomial fit coefficients for fish tail instead of segment angles and tail sum

[vilim]

Has to be verified, but in my investigations a 3rd degree polynomial captures everything interesting and is robust to missing segments. np.polynomial.polyfit is also very fast.

[vigji]

Still, it will add another layer before data are saved, I think most users would not want that. Although one could argue that what is saved already comes from an interpolation?