Extend PSF photometry to simultaneous fitting of multiple images, similar to the extract_stars framework?

Pretty lofty goal, but it seems we're not taking full advantage of the `EPSFStar` vs `LinkedEPSFStar` framework only allowing for fitting of a single image to sources at once. With `constrain_centers` on linked sources, it feels like it should be possible to allow for multiple images to be fit (in parallel or in series), with common knowledge of _position_ of all sources being used to better constrain fluxes...

Fairly big extension, but just a thought I had... Is there a tag for "would be nice down the line" or something? Or a "lofty dream" tag?

Start by reviewing the existing extract_stars framework and the EPSFStar, LinkedEPSFStar, and constrain_centers behavior in the photometry code. Determine how simultaneous multi-image fitting could share source positions, then identify the relevant tests and define completion as fitting multiple images with common positional information and verified flux results.

Written by the indexing model from the issue text.