Homogenize naming of calibration flags

We currently have the following set of names for the calib_* flags that get propagated to source catalogs:

• calib_detected
• calib_psfCandidate
• calib_psfUsed
• calib_psf_reserved (changed from calib_psfReserved on DM-12207 in this commit)
• calib_astrometryUsed
• calib_photometry_used (changed from calib_photometryUsed on DM-12207, in this commit)
• calib_photometry_reserved (changed from calib_photometryReserved on DM-12207 in this commit)

I.e. there is a mixture of underscore and camelCase for the third (final) "descriptor", making it difficult to remember which standard to use when trying to select on a given flag.  These should all be homogenized and, given the recent changes in this direction noted above, I believe the preferred choice is the underscore standard.  The text in afw/doc/table.dox is soon to suggest a preference for this format (DM-14998), in particular that one should use "underscores when you want the prefix to be considered a conceptual group".  Update: now merged and [reads

As such, the official proposal here is to make the following schema field name changes:

• calib_psfCandidate -> calib_psf_candidate
• calib_psfUsed -> calib_psf_used
• calib_astrometryUsed -> calib_astrometry_used

A change in the flag names will affect any current user scripts that select on these flags.  To prevent scripts from breaking, it may be desirable to set a (temporary?) alias (this was not done along with the DM-12207 changes and there was no uproar that I'm aware of, but the use of the calib_psfUsed flag is much more broadly used that any of the others).