Started adding data about the tires.

* I've renamed the old Tire class into ObstacleTire. These tires are
only being used as obstacles, so they don't need all of the
implementation details that the car tires need.
* I've hard-coded the tire size information on the RaceCar class.
* I've started implementing the CarTire class.
I've added the code to the CarTire class that will find the tire's mass and inertia.