Set the ice cloud asymmetry factor for all wavelengths and altitudes
to a float between -1.0 and 1.0.
Please note that this option is only applied if a Henyey-Greenstein
phase function is used but not if an explicit phase function is
defined e.g. with a ic_files. It doesn't make sense to modify only
the first moment of an explicit phase function.
ic_set_gg value
If you use this option in combination with the ice
cloud properties by Fu1996, please make sure that you understand
the explanation of ic_fu_tau.