support custom groups for zproc, e.g. 1x_depth

[sbailey]

This PR adds support for custom groups of exposures for zproc, e.g. to enable 1x_depth processing through zproc itself, instead of running the underlying commands by hand like was done for previous prods. The key update is adding the concept of a user-specified "subgroup" to findfile and zproc to be used for custom groupings. For pernight and cumulative, subgroup defaults to night, and perexp and healpix also work as before.

Examples in /global/cfs/cdirs/desi/users/sjbailey/spectro/redux/jura1x

# Examples for running 1x_depth
desi_zproc --batch --nosubmit -t 80605 \
    --groupname 1x_depth --subgroup 1 -n 20210205 20210130 -e 74781 73702
desi_zproc --batch -q debug -t 80605 \
    --groupname 1x_depth --subgroup 2 -n 20201215 20210205 -e 67975 74782

# Example to confirm that previous default cumulative grouping isn't broken
desi_zproc --batch --nosubmit -t 1000 -n 20210517

The first and last case with --nosubmit generated the slurm file, which I then sourced in an interactive node. The middle case without --nosubmit ran in the debug queue as expected.

This PR will make it much easier to run 1x_depth followup to Jura to evaluate new QuasarNET models for Kibo.

[akremin]

Overall this looks good. I found what appears to be two typos in the unit tests. I also request a name change to the proposed new jobdesc. I'd prefer something like CUSTOMZ or CUSTOMTILE (with no underscore).