Set the aerosol asymmetry parameter for all wavelengths and altitudes
to a constant value 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 aerosol_files. It doesn't make sense to modify only
the first moment of an explicit phase function.
aerosol_set_gg value