Describe the bug
Model._event_generators continues to grow while running a model, and also will contain generators that are exhausted. It is a minor bug, but worth addressing at some point. But since a generator also holds a hard ref to the callable, it might, for example, prevent agents from being garbage collected.

Expected behavior
Whenever a generator is exhausted (or stopped?), it should remove itself from model._event_generators.

proposed direction for fix
There are a couple of directions:

1. Turn ._event_generators into a set, and whenever a generator is exhausted, it removes itself from this set. The only function of _event_generators is to serve as a hard ref to generators, so ordering does not matter. This fix would also imply that the responsibility of adding a generator to schedule_recurring should move from schedule_recurring into the EventGenerator class, so all instances are in the set.
2. Make it the responsibility of the caller of schedule_recurring to maintain a hard ref to the generator. This is tricky for users, so less ideal.
3. To reduce the severity of the leak, we should consider turning self.function into a weakref similar to how it is done for Event, and if the ref becomes None, this should automagically stop the generator.