Let's have Networks and Terrains automatically sync with all Agents in their Environment

[scottpdo]

^ Pretty self-explanatory...

Instead of having their own .addAgent methods, Networks and Terrains, when constructed with an Environment, should just point their .agents array at the Environment's. Then when the Environment adds or removes Agents, they will automatically be synced to the helper in question.

In addition to updating the constructors, Environment's .addAgent and .removeAgent methods will need to be updated to do any cleanup/updates in the helpers (for example, if removing an Agent that has connections in the Network, remove those connections).

Since this will involve API changes, probably should go under a new major release 0.6.0.