[IMP-dev] minor but important API change

The create_* helper functions (such as those for rigid body) now do not add the score state to the model, you must do it yourself. The ScoreState will warn you (a bit cryptically) if it is not added to a model before it is destroyed. Note that it is possible to miss the message if you use C++ and never destroy the object, I'll figure out a way to warn in this case too.