Implement VecEnv

[will-maclean]

A strength of stable-baselines3 is its vector envs, which allow for multiple envs to be executed in parallel. Let's see if we can implement the same functionality.

https://github.com/DLR-RM/stable-baselines3/tree/master/stable_baselines3/common/vec_env

Components:

• create trait
• sub process vec env? (or just use threads??)
• common wrappers

[will-maclean]

May need to act a batch_act(&mut self, obs: Vec<O>) -> Vec<A> to the Agent trait.