ert_diff_down       , &
                            gcm_vert_diff_up         , &
                            surf_diff_type           


  gcm_vert_diff_init : initializes module

  gcm_vert_diff_end : clears memory

  vert_diff : adds tendency (derivative with respect to time)
              due to vertical diffusion to tendencies of a single field -- 
              to be used only if surface flux is not implicitly 
              dependent on surface parameters (it can be implicit
              function of 