It turns out I have to do this to get DM-14155, because obs_sdss relies on objectSizeSS for measureApCorr, which was ported to the new API in DM-9832.

This is the remainder of the work on DM-9832, after I discovered that the CI packages (lsst_stack_demo, lsst_ci) were failing for various reasons. I had to convert two more of the StarSelectors to get CI to pass locally.

Note that some of these PRs will be merged to the DM-9832 epic branch, before being merged to master, as they were based on that branch.

Jenkins run with all relevant tickets: https://ci.lsst.codes/blue/organizations/jenkins/stack-os-matrix/detail/stack-os-matrix/27942/pipeline

Looks good; just one tiny doc comment on one PR.

Does this obsolete obsolete any of DM-9837, DM-9838, and DM-9840?

9838 was done as part of this.

I'm leaving 9837 and 9840 until later because I didn't need to do them to get the existing tests to pass. We'll get them soon enough though. We don't have an "exercise every thing in a registry" style of test, which could be useful to confirm that every item registered follows the API.

Thanks for the quick review.

Merged into 9832 and into master, and done!