This function is design to be used with applyCdfGroups()
on an Affymetrix Mapping (SNP) CDF list structure.
cdfGtypeCelToPQ(groups, ...)list structure with groups.list structure with a single group. The fields in this
groups are in turn vectors (all of equal length) where the elements
are stored as subsequent quartets (PMA, MMA, PMB, MMB) with all
forward-strand quartets first followed by all reverse-strand quartets.
applyCdfGroups().