Metadata propagation for coadds

[sbailey]

This PR

• Adds a SCORES table HDU to coadds, with columns INTEG_COADD_FLUX_B/R/Z, MEDIAN_COADD_FLUX_B/R/Z, and MEDIAN_COADD_SNR_B/R/Z, similar to the scores tables in the frames. The same columns are generated whether or not it is coadded across cameras. Fixes coadds: propagate / recalc SCORES HDU #885.
• Adds INFILnnn header keywords with the input files used for this coadd. Fixes coadds: include what exposures went into them #884.

cd /global/cfs/cdirs/desi/spectro/redux/daily
desi_coadd_spectra -i tiles/70004/20200219/cframe-?0-*.fits \
    -o $SCRATCH/temp/coadd1.fits --coadd-cameras
desi_coadd_spectra -i tiles/70004/20200219/cframe-?0-*.fits \
    -o $SCRATCH/temp/coadd2.fits

and then inspecting the coadd*.fits files to verify that they have SCORES HDUs and INFILnnn keywords.

This PR does not fix the propagation of the FIBERSTATUS column in the fibermap.

@akremin please review. Thanks.

[akremin]

Things run as designed and the code looks good. Merging