Correct logical associated with NW2 tracers

- @klindsay28 spotted two issues for the NW2 tracers
  1. Use of the wrong logical variable
  2. Incorrect comment
- Using the wrong logical meant that the ideal age pacakge was being
  called in addition to the NW2 package, but did not affect the NW2
  tracers themselves.

src/tracer/MOM_tracer_flow_control.F90

@@ -90,7 +90,7 @@ module MOM_tracer_flow_control
   logical :: use_pseudo_salt_tracer = .false.      !< If true, use the psuedo_salt tracer  package
   logical :: use_boundary_impulse_tracer = .false. !< If true, use the boundary impulse tracer package
   logical :: use_dyed_obc_tracer = .false.         !< If true, use the dyed OBC tracer package
-  logical :: use_nw2_tracers = .false.             !< If true, use the ideal age tracer package
+  logical :: use_nw2_tracers = .false.             !< If true, use the NW2 tracer package
   !>@{ Pointers to the control strucures for the tracer packages
   type(USER_tracer_example_CS), pointer :: USER_tracer_example_CSp => NULL()
   type(DOME_tracer_CS), pointer :: DOME_tracer_CSp => NULL()
@@ -267,7 +267,7 @@ subroutine call_tracer_register(HI, GV, US, param_file, CS, tr_Reg, restart_CS)
   if (CS%use_dyed_obc_tracer) CS%use_dyed_obc_tracer = &
     register_dyed_obc_tracer(HI, GV, param_file, CS%dyed_obc_tracer_CSp, &
                              tr_Reg, restart_CS)
-  if (CS%use_nw2_tracers) CS%use_ideal_age = &
+  if (CS%use_nw2_tracers) CS%use_nw2_tracers = &
     register_nw2_tracers(HI, GV, param_file,  CS%nw2_tracers_CSp, tr_Reg, restart_CS)
 
 end subroutine call_tracer_register