Fixed downsampling for x:sum y:point z:point diags

- previous code had averaging instead of summation for
  SPP (x:sum,y:point,z:point) diagnostics
- corrects an issue where these diagnostics were incorrect
  by approximately a factor of 2.
- Orginially found when analyzing the depth-integrated
  temperature advection diagnostic (T_ady_2d)

src/framework/MOM_diag_mediator.F90

@@ -4181,7 +4181,7 @@ subroutine downsample_field_2d(field_in, field_out, dl, method, mask, diag_cs, d
         total_weight = total_weight +weight
         ave = ave+field_in(ii,jj)*weight
       enddo
-      field_out(i,j) = ave/(total_weight+epsilon)  !Avoid zero mask at all aggregating cells where ave=0.0
+      field_out(i,j) = ave  !Masked Sum (total_weight=1)
     enddo ; enddo
   elseif (method == PMP) then
     do j=jsv_d,jev_d ; do i=isv_d,iev_d