Better access to agent listings from python client

[rmm-fcul]

A "list all" in attached simulator would be useful in many ways. Use cases:

• build a population-level position monitor, to compute population-level and individual-level statistics while a simulation is running (see issue #52)
• clear [kill] all objects in simulation (optionally filtered by object) [see issue #20]
• randomise positions of all bees
• attach behavioural scripts to all bees, without needing to know their names ahead of time.