This flag represents a quick way of applying the chi-factor to
CO2 lines (compared to using the SHP flag
+ *SHP section) - it is similar to
the MIX flag (for CO2 line-mixing) in
having no associated driver table sections.
The effect of this flag is to change the default lineshape for any
CO2 lines from Voigt to Voigt x chi-factor, so it can still be over-ridden
by the *SHP section.
It is not necessary to include CO2 as an absorber if this flag is
enabled, but a warning message
will result if it is not included:
'W-INPGAS: CHI flag selected but CO2 not used'
Although in principle including the chi-factor gives a more accurate
representation of pressure-broadened CO2 lines, it is computationally quite
expensive and probably makes little difference at higher altitude where
Doppler broadening starts to dominate.