variable gravity

[WChen-NOAA]

Initial NOAA-GFDL/GFDL_atmos_cubed_sphere#394
The model uses the shallow atmosphere approximation with constant gravity.
This is problematic for a space weather model (WAM) where Ztop/r0 )~10%
The method to resolve this issue is to Implement a scaling factor wherever gravity is applied to account for variations with height:
g(r0/r)^2 while setting var_grav = .true. in the name list
The new gravity as grav_var(i,j,k)