Adds noise distractor options

[ssnl]

The main mechanism to add distractors is utils.EnvElementManager objects, each of which conceptually manages the evolution of a certain element. Its methods reset(), step(), and pre_render() are called in main environment respective places. This allows placing all code related to a single distractor in a single place, making it much more readable.

Additionally, the RoboDesk class is broken into a base class RoboDeskBase and subclasses RoboDesk and RoboDeskWithTV. The base class has very detailed parameters for distractors. The subclasses

1. Specify different xmls (most notably one xml has TV, and one does not), and
2. Use the distractor parameters appropriate for the corresponding scene.
3. Provide with an easy-to-use API for distractors: RoboDesk(..., distractor="all") or ="none" or ={'env_light', 'camera'}.
4. Have different camera viewpoints, specified via CameraSpec.

Notably among EnvElementManagers:

• The CameraManager is created from CameraSpec and is the definitive entry point to create a rendered view of the scene (as it manages camera location).
• The ButtonManager models potential noisy button sensor readings, so elements that change wrt them (i.e., indicator lights on desk, and TV hue) should read from its method get_normalized_button.
• The TVManager loads video files and updates frames to the mujoco model.

This PR is built on top of #3. Let me know how I can better facilitate review.

Finally, if this gets merged, could I be added to readme/acknowledgement/bibtex so that I can help with potential issues people have with the distractor options?