(t and q also used to compute density)

         diff_m -- real, dimension(:,:,:) 
             kinematic diffusivity for momentum (m*2/s)
         diff_t -- real, dimension(:,:,:) 
             kinematic diffusivity for heat, water vapor, tracers (m*2/s)

             shape must conform to those of u, v, t, q
             diff_m(:,:,k) and dsiff_t(:,:,k) are the diffusivities
                 evaluated at the interface between the k-1 and k levels
             the values diff_m(:,:,1) and diff_m(:,:,1) are