Waymo tools and offline dataset extraction

[saulfield]

This PR supersedes #1541 and #1461.

Changes

• Add waymo commands to scl to:
  • Display information about all the scenarios within a TFRecord file
  • Plot the map, and optionally show vehicles with their IDs or animate their trajectories
  • Export a scenario to a SMARTS scenario that can be replayed
• Update the traffic_histories_to_observations.py example to work with any traffic history format (e.g. NGSIM, Waymo), and to output the full Observation structure and set of birds-eye-view images (e.g. for use in offline RL)
• Updates to traffic_histories_vehicle_replacement.py to make it compatible with the new data format

[Gamenot]

Some comments but I think this can go in. Good work.

[Adaickalavan]

1. TRAJECTORY_HANDLES and MAP_HANDLES appear to be the same in waymo_browser.py and waymo_utils.py. Can they be combined?

2. Several functions appear similar between waymo_browser.py and waymo_utils.py, e.g., _plot_trajectories, _get_map_features, _plot_map_features. Can they be combined?

3. Can the gen_smarts_scenario_code function in waymo_utils.py and smarts/waymo/templates/scenario_template.py be combined?

[saulfield]

1. TRAJECTORY_HANDLES and MAP_HANDLES appear to be the same in waymo_browser.py and waymo_utils.py. Can they be combined?

   2. Several functions appear similar between waymo_browser.py and waymo_utils.py, e.g., _plot_trajectories, _get_map_features, _plot_map_features. Can they be combined?

   3. Can the gen_smarts_scenario_code function in waymo_utils.py and smarts/waymo/templates/scenario_template.py be combined?

Thanks @Adaickalavan -- I've implemented all of these suggestions except for the 2nd one, since they are different enough in each tool and I don't think it's worth unifying them at this time. Perhaps in the future though.