Better handle source catalog saving

[schlafly]

Presently if one runs the source catalog via:
strun romancal.steps.SourceCatalogStep image.asdf
one ends up saving the same catalog twice, once from stpipe's behavior returning the catalog and once from
https://github.com/spacetelescope/romancal/blob/main/romancal/source_catalog/source_catalog_step.py#L130
. It looks to me to be a little awkward to get the pipeline handling and step handling right there for a step that returns two objects.

The right behavior is probably to have source catalog always return a tuple with both the catalog and the segmentation image, and to handle saving those by overriding step.save_model following Webb's example here (credit to Brett for pointing this function out).
https://github.com/spacetelescope/jwst/blob/ce9a135f3bc6bba21e95dd47a91fc4d9ba5a1991/jwst/ami/ami_analyze_step.py#L29-L33
See related code in stpipe here:
https://github.com/spacetelescope/stpipe/blob/01f1956f3b3c3750a26790d5217cf909cad4b95a/src/stpipe/step.py#L924

If we did this, we would also want to get rid of the special "return_updated_model" logic in source_catalog and instead update the image metadata in the pipeline.