Closing training environment

[cswinter]

When creating an environment from Python, there needs to be some way to shut down the threads running the apps. Currently, this is done by having the TrainAgent::act method return an Option<Action> rather than an Action directly. When the counterparts to the channels owned by the TrainAgent are dropped, TrainAgent will return a None value which can be handled by sending an AppExit event. Ideally, Agent::act could just return an action directory and app shutdown would be handled automatically without writing additional code. I don't currently know enough about Bevy to know how to make this work. Maybe there could be an entity-gym Bevy plugin that connects to the train agent and handles app shutdown?