• erp5_base: add frozen state and freeze transition in delivery_causality_workflow.
• ERP5/mixin/rule.py: do nothing in _updateSimulation() if causality_state is 'frozen'.

Background :

The purpose I need this is to delete simulation trees (and avoid regeneration of simulation tree of course) for old enough documents whith we no longer want to handle at all. Since the consistency with simulation is tracked by causality_state, I choose it as the place to declare no-longer-simulated. And I choose frozen as the state id and freeze as the transition id and also give Add portal content permission to nobody and Modify portal content permission to Manager only.

But there may be use cases where we simply want to abandon simulation but still wants to modify or add sub documents etc. If so, the id of the new state and the transition should be different (but what ?).