rds) 
             shape must conform to first two dimensions of xi
             Units = [factor]*[xi]*kg/(m**2 s)
             Flux should be evaluated with atmospheric fields 
                     from time i   for two-time level schemes
                     from time i-1 for leapgrog
             If dflux_datmos is non-zero, input flux is modified by 
             the implicit correction 
                 flux(i+1) = flux(i or i-1) + delt*dflux_datmos*dt_xi(:,:,N)
                 where dt_xi is the fina