dded 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 contributed to 
                the tendency before, and which after, this call

             Units of dt_xi are [xi]/s where [xi] are units of xi

         dt_tr -- real, dimension(:,:,:,:)
             Tracer tendency (see above)  size(dt_tr,4) = number of tracers
	     
     output: 
 