Bugfix: reference height offset for SAL using bpa

Recover reference height offset (Z_ref) in calculating bottom pressure
for self-attraction and loading in Boussinesq mode.

src/core/MOM_PressureForce_FV.F90

@@ -1242,7 +1242,7 @@ subroutine PressureForce_FV_Bouss(h, tv, PFu, PFv, G, GV, US, CS, ALE_CSp, p_atm
     if (CS%sal_use_bpa) then
       !$OMP parallel do default(shared)
       do j=Jsq,Jeq+1 ; do i=Isq,Ieq+1
-        pbot(i,j) = pa(i,j,nz+1) - GxRho_ref * e(i,j,nz+1)
+        pbot(i,j) = pa(i,j,nz+1) - GxRho_ref * (e(i,j,nz+1) - G%Z_ref)
       enddo ; enddo
       call calc_SAL(pbot, e_sal, G, CS%SAL_CSp, tmp_scale=US%Z_to_m)
     else