BT_rem_[uv] checksum as scalar pair

BT_rem_[uv], which applys a fractional weight to the barotropic
velocity, was incorrectly computed as a vector pair, which computes the
incorrect checksum.

This patch flags this as a scalar pair, which removes any discrepancy in
standard and rotated debug output.

src/core/MOM_barotropic.F90

@@ -2173,7 +2173,8 @@ subroutine btstep(U_in, V_in, eta_in, dt, bc_accel_u, bc_accel_v, forces, pbce,
                     scale=US%L_T_to_m_s*US%s_to_T)
       call uvchksum(trim(mesg)//" BT_force_[uv]", BT_force_u, BT_force_v, CS%debug_BT_HI, haloshift=iev-ie, &
                     scale=US%L_T_to_m_s*US%s_to_T)
-      call uvchksum(trim(mesg)//" BT_rem_[uv]", BT_rem_u, BT_rem_v, CS%debug_BT_HI, haloshift=iev-ie)
+      call uvchksum(trim(mesg)//" BT_rem_[uv]", BT_rem_u, BT_rem_v, CS%debug_BT_HI, &
+                    haloshift=iev-ie, scalar_pair=.true.)
       call uvchksum(trim(mesg)//" [uv]bt", ubt, vbt, CS%debug_BT_HI, haloshift=iev-ie, &
                     scale=US%L_T_to_m_s)
       call uvchksum(trim(mesg)//" [uv]bt_trans", ubt_trans, vbt_trans, CS%debug_BT_HI, haloshift=iev-ie, &