Make the stop points simulation agents

[leo-desbureaux-tellae]

Problem exposition

Stop points are an exception in Starling, as they are stored in the agent population as if they were agents. They are stored here because it is convenient to have the global collection of stop points instead of each operator having its collection of stops.

Solution proposed

It could be useful to make StopPoint inherit from SpatialAgent. They are not so different from stations, and could even also use a SimPy store for their agent queues.

It would also allow tracing events for stop points and compute specific KPIs.

This implies many modifications on how stop points are managed, at many different points in Starling.