Statmorph

- Users can now perform _double_ 2D Sersic fits by including the option `include_doublesersic = True`.
- Users can (optionally) customize the fits via the arguments `doublesersic_model_args` and `doublesersic_fitting_args`.
- Furthermore, the option `doublesersic_tied_ellip = True` can be used to ensure that both components have the same ellipticity and position angle, if desired.
- The quality flags `flag_sersic` and `flag_doublesersic` now take values 0-4, just like `flag` (see v0.5.0 release notes).
- Added reduced chi^2 statistics (`sersic_chi2_dof` and `doublesersic_chi2_dof`) to measure goodness of fit.
- Added an example notebook about double Sersic fitting to the documentation.
- Updated tutorial.

- Added support for constrained Sersic fits via the `sersic_model_args` keyword.
- The input argument `sersic_maxiter` is deprecated. Please use `sersic_fitting_args` instead.
- Default value of `cutout_extent` increased from 1.5 to 2.5 (more "space" to make measurements by default, although this also depends on the input segmap).

The "bad measurement" flag now takes values between 0 and 4 (formerly 0 and 1), as explained in the [documentation](https://statmorph.readthedocs.io/en/latest/description.html#output). In summary, a flag of 1 now indicates mild issues with the measurements, while a flag of 2 is reserved for more serious problems. In practice, users of previous versions of statmorph need to know the following when upgrading to v0.5.0:

- The statement `flag == 0` has the same effect as before, since a flag of 0 indicates good measurements.
- The statement `flag != 1` should be replaced with `flag == 0` (to avoid erroneously selecting `flag == 2` cases).
- The statement `flag == 1` (or `flag != 0`) should be replaced with either `flag >= 1` (to restore the functionality of previous versions) or with `flag == 2`, which should result in a smaller fraction of "bad" measurements.
- Users who are dealing with a high fraction of "bad" measurements should consider replacing `flag == 0` with the less strict `flag <= 1` condition.
- The `flag_catastrophic` property was removed and is now represented by `flag == 4` (note that all calculations are aborted when this happens).

- Various small updates.

- Implement suggestions from Astropy review.
- Introduce "verbose" mode (less warnings by default).
- Other small changes.

- Compatibility with photutils 1.0.
- Change default value of segmap_overlap_ratio (more permissive => less "bad measurement" flags).
- Other small changes.