Disable adjustment of BT mass source in RK2b corrector (#6)

Adds analogous code to MOM_dynamics_split_RK2b.F90 that is identical to the
code that was added to MOM_dynamics_split_RK2.F90 to avoid readjusting the
barotropic mass source to try to reconcile the baroclinic and barotropic sea
surface heights.  By default all answers are bitwise identical, but the runtime
parameter BT_ADJ_CORR_MASS_SRC will not appear in the MOM_parameter_doc files
for cases with SPLIT_RK2B set to True.

Author: Hallberg-NOAA

src/core/MOM_dynamics_split_RK2b.F90

@@ -154,6 +154,9 @@ module MOM_dynamics_split_RK2b
                                                       !! effective summed open face areas as a function
                                                       !! of barotropic flow.
 
+  logical :: BT_adj_corr_mass_src !< If true, recalculates the barotropic mass source after
+                                  !! predictor step. This should make little difference in the
+                                  !! deep ocean but appears to help for vanished layers.
   logical :: split_bottom_stress  !< If true, provide the bottom stress
                                   !! calculated by the vertical viscosity to the
                                   !! barotropic solver.
@@ -811,9 +814,11 @@ subroutine step_MOM_dyn_split_RK2b(u_av, v_av, h, tv, visc, Time_local, dt, forc
   ! used in the next call to btstep.  This call is at this point so that
   ! hp can be changed if CS%begw /= 0.
   ! eta_cor = ...                 (hidden inside CS%barotropic_CSp)
-  call cpu_clock_begin(id_clock_btcalc)
-  call bt_mass_source(hp, eta_pred, .false., G, GV, CS%barotropic_CSp)
-  call cpu_clock_end(id_clock_btcalc)
+  if (CS%BT_adj_corr_mass_src) then
+    call cpu_clock_begin(id_clock_btcalc)
+    call bt_mass_source(hp, eta_pred, .false., G, GV, CS%barotropic_CSp)
+    call cpu_clock_end(id_clock_btcalc)
+  endif
 
   if (CS%begw /= 0.0) then
     ! hp <- (1-begw)*h_in + begw*hp
@@ -1350,6 +1355,11 @@ subroutine initialize_dyn_split_RK2b(u, v, h, tv, uh, vh, eta, Time, G, GV, US,
   call get_param(param_file, mdl, "SPLIT_BOTTOM_STRESS", CS%split_bottom_stress, &
                  "If true, provide the bottom stress calculated by the "//&
                  "vertical viscosity to the barotropic solver.", default=.false.)
+  call get_param(param_file, mdl, "BT_ADJ_CORR_MASS_SRC", CS%BT_adj_corr_mass_src, &
+                 "If true, recalculates the barotropic mass source after "//&
+                 "predictor step. This should make little difference in the "//&
+                 "deep ocean but appears to help for vanished layers. If false, "//&
+                 "uses the same mass source as from the predictor step.", default=.true.)
   ! call get_param(param_file, mdl, "FPMIX", CS%fpmix, &
   !                "If true, apply profiles of momentum flux magnitude and direction.", &
   !                default=.false.)