Add multi-agent functionality to ULTRA episodes function

[christianjans]

Currently, the Episode object used to record agent data during training and evaluation only supports the recording of a single agent's data.

This change will allow the Episode object to record the training and evaluation data of multiple agents.

[AlexLewandowski]

Would it be more convenient to have a default AGENT_ID arg in episodes, like 007, so that we won't have to change every instance of episodes?

[christianjans]

Yeah, good point! The extra agent_ids parameter was inconvenient, this new version gets rid of it. The new version allows whatever is using episodes to track whichever agent without having to specify an agent_ids parameter in episodes(...).