HSC backport: multiband processing for coadds

This issue includes transferring changesets from many HSC issues:

• HSC-1060
• HSC-1064
• HSC-1065
• HSC-1061

Most of this is in multiBand.py in pipe_tasks, but there are scattered changes elsewhere (including updates to camera mappers to include the new datasets, for which we'll need to modify more than just obs_subaru).

However, before we make these changes, we'll need to open an RFC to gather comments on the design of this task. We should qualify there that this is not a long-term plan for consistent multiband processing (which we'll be starting to design on DM-1908), but a step towards better processing in the interim.

Note: while I've assigned this to Lauren MacArthur, as I think it will be very helpful for her to get familiar with this code by doing the transfers, the RFC will have to involve a collaboration with Jim Bosch, Paul Price, and Bob Armstrong, as we can't expect someone who wasn't involved in the design to be able to write a document justifying it.