Group coaddition is performed by taking the average of all of
the sky-subtracted pairs in the group. The header values for
airmass and UT time at the end of observation are updated in
the group frame.
[_PAIR_REDUCTION_COADD_TO_GROUP_]
In polarimetry mode multiple group files are created, one for
each waveplate position. Airmass and UT time header values
are also updated.
[_PAIR_REDUCTION_COADD_TO_GROUP_POL_]
ORAC-DR -- spectroscopy data reduction