l (output) tendency and 
                 N = size(dt_xi,3) is the lowest model level

         dt_xi -- real, dimension(:,:,:)
             Tendency of xi
             shape must conform to that of xi
             input is the tendency due to all other processes
                 that have been added prior to this call
             output will be the tendency modified by the effects of diffusion
             Because the algorithm is implicit, the final tendency
                depends on which other process