Eli Rykoff: do you think you can tackle this review? It's of order 500 lines, across 7 packages (the afw branch and PR aren't showing up in Jira, so I've linked it below).
Here's the missing afw PR: https://github.com/lsst/afw/pull/420
Jim Bosch: I've modified the meas_mosaic files in-place for now. I'm talking with Hsin-Fang Chiang about running this meas_mosaic branch on some PDR data and replacing the persisted files to check that that all works.
Note that this change means that old PhotoCalibs cannot be read. I've filed
DM-16830 to give versioning to PhotoCalib, but for this particular change we'd have to also version the underlying BoundedFields. Fortunately, PhotoCalib isn't in significant use right now, so this should not disrupt anything.