Migration guide: Add note about removing Agents from the model

[EwoutH]

Add note about removing Agents from the model to the Migration guide.

It now reads:

5. Instead of self.schedule.add() and self.schedule.remove(), agents are now automatically added to and removed from the model's AgentSet (model.agents) when they are created or removed.
   • You still need to remove the Agent itself. Use Agent.remove() for that. Often you would:
     • Replace self.schedule.remove(agent) with agent.remove() (in the Model)
     • Replace self.model.schedule.remove(self) with self.remove() (in the Agent)

[Corvince]

Not related to the PR itself, but I find the semantics here a bit confusing. Why should I have to call agent.remove if I want to remove it from model.agents? And why can't I do agent.add()? I think we should actually remove agent.remove because of its unclear semantics. Thus, removing an agent can mean two things

1. del agent, in which case it is automatically removed from model.agents (right?)
2. model.deregister_agent(agent). In case I want to remove it from the model, but keep it around, maybe to add it back later via model.register_agent. This would mean we should keep those methods public (Performance enhancements for Model.agents #2251 (comment))

[quaquel]

del agent, in which case it is automatically removed from model.agents (right?)

I don't think that will work. Del removes a reference from a namespace nothing more and there is AFAIK (and just checked) no magic method that can used to change the behavior of del. __del__ and __delete__ are not tied to del and do subtle different things.