Statmorph

- Update to photutils v2.0.

- Change `rms_asymmetry` to `rms_asymmetry2`, since this quantity actually represents the _square_ of A_rms.

- Implement RMS asymmetry (Sazonova et al. 2024).

- More robust search for asymmetry center.
- Attempt Sersic fit even when basic measurements were not successful (flag >= 2).

- Make statmorph slightly more memory efficient.

- An improved initial guess for the double Sersic fit is now obtained by performing single Sersic fits to to the inner and outer regions of the source of interest, which are assumed to be separated by an ellipse of a given size.
- The separating ellipse is by default twice the half-light ellipse, but this can be controlled by the user with the argument `doublesersic_rsep_over_rhalf`
- The Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) are now calculated for the single and double Sersic fits.