shr_flux_mod update to use shr_wv_sat_mod

[billsacks]

Moving from ESMCI/cime#854

From @jedwards4b original comment in that issue (from 2016):

This issue is moved from CESM-DEVELOPMENT issue 39.
shr_flux_mod should use the saturation vapor pressure from shr_wv_sat_mod instead of
having it's own method.
Any of the common approximation methods will give effectively the same answer at reasonable temperatures/pressures over the ocean, and this is not performance-critical code that needs optimization. Still, it's better practice to call the standard routine than to have a series of duplicated/hard-coded formulas, so this should probably be cleaned up some time.

The one caveat is that there's a factor of 0.98 applied to the humidity, which roughly adjusts for the ocean salinity. That factor needs to be kept, in the subroutines where it is present to begin with.