input flux
                                  

         dt_u, dt_v, dt_t, dt_q -- real, dimension(:,:,:)
             Tendency of u, v, t, q
             shape must conform to that of u, v etc
             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 processes have c