EventBase.part_of should be acyclic

[cogat]

Two events A and B can be made to be part_of one another, ie event A can indirectly (possibly even directly) be its own ancestor. This can cause recursion-depth errors, when trying to traverse relationships.

However, in the ACMILabs fork, we've also made part_of a many-to-many, since an event can be part_of many parent events. So the event structure should be a directed acyclic tree, rather than an mptt-type tree where every Event only has one parent.