Extend Gen3 butler support for obs_cfht

DM-21860 adds basic Gen3 butler support for obs_cfht (sufficient for ingest), but we'll need to expand that to get to the point where we can run single-frame processing (IsrTask, CharacterizeImageTask, and CalibrateTask) via Gen3 on it.  That will include, at least:

• implementing writeCuratedCalibrations in the instrument class;
• specializing (and modifying, as necessary) the instrument-specific hooks for the gen2to3 tool, at least to the point where we can write scripts like the ones in ci_hsc_gen3/bin.src to make a fully-usable Gen3 repo.