Gen2 repositories already have SkyPix datasets (reference catalogs) that use possibly-different levels of HTM.  That'll be tricky to support with our (probably naive) plan to have a single SkyPix pixelization and level in each Registry.  Come up with a design that solves that problem, and at least think about whether HiPS/MOC datasets pose a similar problem (they probably do).

Options include:

• RFC rewriting reference catalogs to a common level.
• Have a common level for all join tables, but allow Datasets to use other levels (with either another join table or an in-database function to relate them).

We also need to think about how to deal with HTM vs. HEALPix; we need support for HTM for reference catalogs now (and already have the necessary low-level code in sphgeom), but we know we'll need to do HEALPix in the future.  That will probably require adding HEALPix to sphgeom (backed by third-party HEALPix libraries, but still not trivial), which we'd rather not do now (not part of this ticket).  But once HEALPix is available, do we do a big-bang switch to it and drop HTM, or do we design a system that permits both to exist within the same registry?

Outputs: write up a proposed design (Confluence or DMTN), and RFC it.