Bugfix in double diffusion

In salt fingering case, I was setting Tdiff_out = 0.7*Sdiff_out for the entire
column rather than just the level where  the stratification parameter indicated
salt fingering. Easy fix: add (k) indices to the arrays.

src/shared/cvmix_ddiff.F90

@@ -405,7 +405,7 @@ subroutine cvmix_coeffs_ddiff_low(Tdiff_out, Sdiff_out, strat_param_num,    &
             CVmix_ddiff_params_in%ddiff_exp1)**CVmix_ddiff_params_in%ddiff_exp2
           Sdiff_out(k) = CVmix_ddiff_params_in%kappa_ddiff_s*ddiff
         end if
-        Tdiff_out = Sdiff_out*0.7_cvmix_r8
+        Tdiff_out(k) = Sdiff_out(k)*0.7_cvmix_r8
       end if
       if ((strat_param_num(k).ge.strat_param_denom(k)).and.                   &
           (strat_param_num(k).lt.cvmix_zero)) then