Set the water 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 wc_files. It doesn't make sense to modify only
the first moment of an explicit phase function.
wc_set_gg value
This option is useful only for monochromatic
calculations or in wavelength regions where the optical
properties of water clouds can be considered constant,
e.g. the ultraviolet range.