desi_group_spectra header propagation cleanup

[sbailey]

This PR fixes the desi_group_spectra large number of warnings from incorrectly merging per-exposure keywords reported in #2200.

The vast majority of keywords in the frame FIBERMAP HDUs are inherited from the raw data, and either conflict when combined (humidities, temperatures, exposure times...) or are fairly meaningless for a fibermap even if they don't conflict (CCD dimensions, clock settings...). This PR treats it as an opt-in list to propagate: by default only

    keep_keywords = ['SURVEY', 'PROGRAM', 'EXTNAME', 'LONGSTRN',
                     'INSTRUME', 'OBSERVAT', 'OBS-LAT', 'OBS-LONG', 'OBS-ELEV', 'TELESCOP']

and if onetile=True it also propagates keywords related to the input fiberassignment, which apply to all observations of a single tile but not coadds across tiles (healpix).

I added some basic unit tests to check this functionality, and also tested by rerunning two Jura cases in /global/cfs/cdirs/desi/users/sjbailey/dev/mergehdr:

import desispec.scripts.group_spectra

desispec.scripts.group_spectra.main(['--inframes', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240118/00219813/cframe-b0-00219813.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240118/00219813/cframe-r0-00219813.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240118/00219813/cframe-z0-00219813.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240128/00222119/cframe-b0-00222119.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240128/00222119/cframe-r0-00222119.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240128/00222119/cframe-z0-00222119.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240128/00222120/cframe-b0-00222120.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240128/00222120/cframe-r0-00222120.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240128/00222120/cframe-z0-00222120.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240128/00222121/cframe-b0-00222121.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240128/00222121/cframe-r0-00222121.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20240128/00222121/cframe-z0-00222121.fits.gz', '--outfile', 'spectra-0-3884-thru20240128.fits.gz', '--coaddfile', 'coadd-0-3884-thru20240128.fits', '--onetile', '--header', 'SPGRP=cumulative', 'SPGRPVAL=20240128', 'NIGHT=20240128', 'TILEID=3884', 'SPECTRO=0', 'PETAL=0'])

desispec.scripts.group_spectra.main(['--inframes', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20210519/00089278/cframe-b7-00089278.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20210519/00089278/cframe-r7-00089278.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20210519/00089278/cframe-z7-00089278.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20210519/00089278/cframe-b8-00089278.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20210519/00089278/cframe-r8-00089278.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20210519/00089278/cframe-z8-00089278.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20210519/00089280/cframe-b2-00089280.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20210519/00089280/cframe-r2-00089280.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20210519/00089280/cframe-z2-00089280.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230808/00189277/cframe-b0-00189277.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230808/00189277/cframe-r0-00189277.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230808/00189277/cframe-z0-00189277.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230808/00189277/cframe-b9-00189277.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230808/00189277/cframe-r9-00189277.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230808/00189277/cframe-z9-00189277.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230815/00190734/cframe-b0-00190734.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230815/00190734/cframe-r0-00190734.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230815/00190734/cframe-z0-00190734.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230815/00190734/cframe-b9-00190734.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230815/00190734/cframe-r9-00190734.fits.gz', '/global/cfs/cdirs/desi/spectro/redux/jura/exposures/20230815/00190734/cframe-z9-00190734.fits.gz', '--outfile', 'spectra-main-dark-10000.fits.gz', '--coaddfile', 'coadd-main-dark-10000.fits', '--healpix', '10000', '--header', 'SURVEY=main', 'PROGRAM=dark'])

(apologies for the very long command lines; those are taken directly from the Jura logs except for editing the output directory location).

The spectra*.fits.gz and coadd*.fits files in /global/cfs/cdirs/desi/users/sjbailey/dev/mergehdr can be compared to the versions in jura/tiles/cumulative/3884/20240128 and jura/healpix/main/dark/100/10000 .

@segasai please check.

[segasai]

Looks fine to me, other than a minor comment that I left.