if SALT_PLUME_VOLUME is undef, keep the call to SALT_PLUME_TENDENCY_APPLY_T (which is empty in this case).
if SALT_PLUME_VOLUME is undef, keep the call to SALT_PLUME_TENDENCY_APPLY_T
(which is empty in this case).

altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h