Slight refactoring of NTT code to prep for additional v3 changes

[nathan-weinberg]

Minor changes made:

• LICENSE and README updates
• Added a bunch of comments
• Renamed a few functions with references to cars changed to vehicles since Silver Line buses are tracked now as well
• Renamed data function to trains since it fetches train data

Less minor changes made:

• Retooled some of the logic in vehicle_data_for_routes() and vehicle_array_is_new()

The idea behind these changes was to preserve the test_mode functionality while ensuring that data for all trains in the tracker was accurate - previous logic defined all trains as new when test_mode was active - current logic should still include all trains in test_mode but their JSON data should accurately show whether or not they are new

• Removed references to newOnly in getTrainRoutePairsForLine()

Not going to lie, I only did this b/c it was the only way to get test mode to work with the changes I made described above. I am not sure why this was needed. Please double check this.

I did test these changes and trains are tracking properly, BUT I am not sure how this will affect interactions with the database, SO we should definitely consider that before merging this PR.

[austinjpaul]

Thanks so much for doing this cleanup!
A couple things caught my eye, mainly in mbta_api.py. Of course, these just reflect my opinions and i'm happy to be overridden on any of them.

[idreyn]

Looks good! Very sorry it took me this long to review. Thanks for taking this on!