DM-36408: Add Coadd PSF and multiband source fitting tasks

[taranu]

These tasks operate on coadd images and are meant to be run sequentially, in contrast to the old monolithic fitting MultibandFitTask in fit_multiband.py, which did both PSF and source fitting in one task. Also, unlike the old task (which will be deprecated soon), the outputs are astropy tables rather than SourceCatalogs. The structure is otherwise similar, with the actual work done by a configurable subtask (not included).

[fred3m]

Just a few comments and minor changes requested. Although there seems to be a lot of duplicate code between the different tasks, and I wonder if you could abstract more of it out?

[taranu]

Just a few comments and minor changes requested. Although there seems to be a lot of duplicate code between the different tasks, and I wonder if you could abstract more of it out?

About the duplication - I accidentally left in a couple of unnecessary methods in the PSF fitting task. Other than the base CatalogExposure dataclass that I left in fit_multiband.py, I don't think there's anything beneficial to be shared with the multiband fitting task.

fit_coadd_multiband.py was indeed adapted from fit_multiband.py and some of the code is identical, but I plan to just axe everything but CatalogExposure in fit_multiband.py. I'm not sure if I need to formally deprecate it considering that the only two implementations of the subtasks are in lsst-dm repos, but I will if necessary.

[fred3m]

No, I don't think that you need to deprecate lsst-dm code.

[taranu]

I meant deprecating the MultibandFitTask in fit_multiband.py, here in pipe_tasks, which is not part of any pipelines in part because there are no implementations of MultibandFitSub[Config/Task] in lsst, only in lsst-dm.

[fred3m]

Right, sorry, that's what I meant too. Even though the task is in pipe_tasks, it requires code from lsst-dm, so I feel like it would be silly to deprecate (effectively) unused code.