icedrv_RunMod: add support for disabling thermodynamics

As noted in the previous commit, 'ktherm = -1' is documented as
disabling the thermodynamics in the Icepack documentation since 2b27a78
(Update documentation including namelist tables (CICE-Consortium#322), 2020-06-05),
although this capability does not exist in Icepack standalone.

Update icedrv_RunMod::ice_step to call some subroutines only if the
thermodynamics is enabled ('ktherm >= 0'). Namely, let prep_radiation,
biogeochemistry, step_therm2 and step_radiation only be called when
thermodynamics is enabled, while letting the rest of the subroutines
always be called. As noted in the previous commit, 'step_therm1' in
particular has to be always called since the computation of the
atmospheric stresses are done in that subroutine.

configuration/driver/icedrv_RunMod.F90

@@ -102,6 +102,7 @@ subroutine ice_step
       use icedrv_flux, only: init_history_therm, init_history_bgc, &
           daidtt, daidtd, dvidtt, dvidtd, dagedtt, dagedtd, init_history_dyn
       use icedrv_history, only: history_format, history_write
+      use icepack_parameters, only: ktherm
       use icedrv_restart, only: dumpfile, final_restart
       use icedrv_restart_bgc, only: write_restart_bgc
       use icedrv_step, only: prep_radiation, step_therm1, step_therm2, &
@@ -147,7 +148,7 @@ subroutine ice_step
       ! Scale radiation fields
       !-----------------------------------------------------------------
 
-      if (calc_Tsfc) call prep_radiation ()
+      if (calc_Tsfc .and. ktherm >= 0) call prep_radiation ()
 
 !      call icedrv_diagnostics_debug ('post prep_radiation')
 
@@ -156,8 +157,10 @@ subroutine ice_step
       !-----------------------------------------------------------------
 
       call step_therm1     (dt) ! vertical thermodynamics
-      call biogeochemistry (dt) ! biogeochemistry
-      call step_therm2     (dt) ! ice thickness distribution thermo
+      if (ktherm >= 0) then
+         call biogeochemistry (dt) ! biogeochemistry
+         call step_therm2     (dt) ! ice thickness distribution thermo
+      endif
 
       ! clean up, update tendency diagnostics
       offset = dt
@@ -206,7 +209,7 @@ subroutine ice_step
       ! albedo, shortwave radiation
       !-----------------------------------------------------------------
 
-      call step_radiation (dt)
+      if (ktherm >= 0) call step_radiation (dt)
 
       !-----------------------------------------------------------------
       ! get ready for coupling and the next time step