After a bit of clean-up, this is the final patch I intend to commit. In addition to what was discussed earlier, this also introduces a new MoL schedule group MoL_PostStepModify:
Phyics thorns can apply enforce constraints in PostStepModify. The difference between PostStep and PostStepModify is that PostStep is scheduled at many other occasions, whereas the PostStepModify is only scheduled during evolution.
Please comment.
-erik