Change structure of repo

[nandantumu]

Some observation on the code structure (in case we want to change it in this pr):

• the Track entity does not depend on the gym.Env, so it could make sense to move it outside envs
• same for the Renderer and all the rendering code.
• most of the files in the envs implement the simulator. A separation between simulator and gym API might look nice.

I would suggest the following structure:

• track
• rendering
• envs
  • simulator: dynamics_models, collision_models, laser_models, base_classes, etc. (all code that should not depend on the gym)
  • f110_env.py: the actual gym env
  • action: gym implementation of action spaces
  • observation: gym implementation of obs spaces

Originally posted by @luigiberducci in #118 (review)