coadd cosmics_nsig default cleanup

[sbailey]

This PR moves "ownership" of the coadd cosmics_nsig=4.0 default from individual scripts into the desispec.coaddition.coadd() function itself. Previously desispec.scripts.coadd_spectra had an argparse default of 4.0, while desispec.scripts.group_spectra used the different default (0.0) from desispec.coaddition.coadd, resulting in coaddition differences from desi_group_spectra ... --coaddfile coadd.fits vs. desi_coadd_specra ... --outfile coadd.fits.

This PR is into the desi_zproc branch (not main) because it is needed for final verification testing that that branch produces the same answer with refactored script workflow.

Tested with:

$> desi_group_spectra \
--inframes \
    exposures/20201215/00067972/cframe-z0-00067972.fits.gz \
    exposures/20201215/00067972/cframe-b0-00067972.fits.gz \
    exposures/20201215/00067972/cframe-r0-00067972.fits.gz \
    exposures/20201215/00067973/cframe-z0-00067973.fits.gz \
    exposures/20201215/00067973/cframe-b0-00067973.fits.gz \
    exposures/20201215/00067973/cframe-r0-00067973.fits.gz \
    exposures/20201215/00067974/cframe-z0-00067974.fits.gz \
    exposures/20201215/00067974/cframe-b0-00067974.fits.gz \
    exposures/20201215/00067974/cframe-r0-00067974.fits.gz \
    exposures/20201215/00067975/cframe-z0-00067975.fits.gz \
    exposures/20201215/00067975/cframe-b0-00067975.fits.gz \
    exposures/20201215/00067975/cframe-r0-00067975.fits.gz \
    exposures/20201215/00067987/cframe-z0-00067987.fits.gz \
    exposures/20201215/00067987/cframe-b0-00067987.fits.gz \
    exposures/20201215/00067987/cframe-r0-00067987.fits.gz \
--outfile temp/spectra-0-80605-20201215.fits.gz \
--coaddfile temp/coadd-0-80605-20201215-AAA.fits \
--onetile --header SPGRP=pernight SPGRPVAL=20201215 NIGHT=20201215 TILEID=80605 SPECTRO=0 PETAL=0

$> desi_coadd_spectra --onetile --nproc 16 -i temp/spectra-0-80605-20201215.fits.gz -o temp/coadd-0-80605-20201215-BBB.fits

$> fitsdiff temp/coadd-0-80605-20201215-AAA.fits temp/coadd-0-80605-20201215-BBB.fits 

data are identical; headers differ due to timestamps and input filenames.