add script to run nightly redshifts per tile

[sbailey]

Adds a script desi_nightly_redshifts to run coadds and redshifts per tile per night (issue #889). This is intended to be run from a batch job (the script itself runs srun multiple times), or to have it create a batch script and submit it:

desi_nightly_redshifts --batch YEARMMDD

will submit a batch script to the realtime queue to run:

• desi_group_tileframes
• desi_coadd_spectra
• rrdesi (redrock)

It auto-recognizes what tiles have already been run, so you can delete coadds/zbest/redrock files in $DESI_SPECTRO_REDUX/$SPECPROD/tiles/{TILEID}/{NIGHT}/ and resumit and it will redo whatever is left.

Things that could be better, but let's do that as a future PR:

• options to specify queue
• exit with meaningful error message if running without --batch from a login node

@kremin please test on 20200226 when you re-run sp6.

[akremin]

I was able to run on night 20200226 without issue. Correctly identified and skipped existing files and only reduced the missing data.