Heat/Salt conservation issue in MEKE

[gustavo-marques]

I've noticed that TOTAL HEAT and SALT are not conserved when USE_MEKE = True and SINGLE_STEPPING_CALL = False, see screenshot below. This suggests that MEKE is probably not designed to work when dynamics and thermodynamics are advanced with separate calls. If that's the case, we should add a FATAL error when these options are mutually selected.