Add physics engine

[kevinburke]

Some notes from building coasters in the game to experiment. All numbers here assume acceleration is a constant. These are using a 2-car mine train.

• It's a slightly easier problem! We can easily approximate it by only looking at pieces where the elevation delta changes.
• Cars on a flat track lose speed. Maybe about 1mph every 10 track pieces, maybe lose speed faster if the initial speed is higher. However, a car will never stop rolling on a flat track, there's some minimum speed that cars have.
• 25 degree down: Gains 7/5 mph per unit traveled.
• 60 degree down: Gains 7mph per unit traveled.
• Large helix down: Gains speed between 1 and 1.5mph per unit traveled. Tested 9 track pieces max speed is 30mph, 3 track pieces max speed is 21 mph. (unit is 1 helix, half a diameter)
• Small helix down: Gains 7/4 speed per unit traveled. Tested 9 track pieces 34mph, 5 track pieces 27mph (unit is 1 helix, half a diameter).