enable zproc.main to be called from other scripts

[sbailey]

This PR provides two changes to desispec.scripts.zproc.main to enable it to be called from other scripts:

• Return an integer error code instead of calling sys.exit directly. Previously even upon success, it would call sys.exit(0) instead of returning to the calling function. The command line script bin/desi_zproc already includes sys.exit(zproc.main()) so that did not need to be changed.
• Generate the command line from input args or sys.argv. This is needed for the --batch option, which previously assumed the command line was coming only from sys.argv which isn't the case if a wrapper script is calling zproc.main with a list of strings.
  • Caveat: if the wrapper script calls zproc.main with an args.Namespace object, this will still incorrectly generate the command line from sys.argv. That's the same as what the current code does, i.e. this PR doesn't solve all cases, but it at least provides a workaround via supporting passing a list of strings to main.

This was motivated by Loa testing wanting to batch submit an individual tile/nights without having to call the command line script and re-import packages etc every time (slow). An alternative would have been to use desi_tile_redshifts and desi_healpix_redshifts instead, but I was mostly done with this update before realizing that from talking with @akremin, and I think these are useful functionality updates anyway.

[coveralls]

coverage: 30.197%. remained the same
when pulling d1a727c on zproc_workflow
into bd7fd13 on main.

[akremin]

Thanks, as stated there are other existing ways to create N>1 desi_zproc batch scripts, but it's useful to be able to do it directly from desi_zproc --batch.

I have one comment inline to simplify an exit.

[akremin]

Thanks for the quick turnaround. Once the checks are finished, I'll merge.