1.15.0

• Bug fixes to KAZ, pistonball, multiwalker, cooperative pong. Versions bumped.
• Removed logo from gather, version bumped.
• Added FPS attribute to all environments to make rendering easier.
• Multiwalker now uses pygame instead of pyglet for rendering
• Renamed to_parallel and from_parallel to aec_to_parallel and parallel_to_aec
• Added is_parallelizable metadata to ensure that the aec_to_parallel wrapper is not misused
• Fixed the API tests to better support agent generation