New continuous control environment: racetrack-v0

This release introduces additional content:

• a new continuous control environment, racetrack-v0, where the agent must learn to steer and follow the tracks, while avoiding other vehicles
• a new "on_road" layer in the OccupancyGrid observation type, which enables the observer to see the drivable space
• a new "align_to_vehicle_axes" option in the OccupancyGrid observation type, which renders the observation in the local vehicle frame
• a new DiscreteAction action type, which discretizes the original ContinuousAction type. This allows to do low-level control, but with a small discrete action space (e.g. for DQN). Note that this is different from the DiscreteMetaAction type, which implements its own low-level sub-policies.
• new example scripts and notebooks for training agents, such as a PPO continuous control policy for racetrack-v0.
• updated documentation