Entity color options

[ewei2406]

Added vertex coloring to the following meshes:
flyer-bus, new-flyer-bus, light-rail-vehicle, sanfrancisco-cablecar, box-truck, city-bus, food-trailer, isuzu-truck, sedan, sedan-taxi, self-driving-cruise-car, suv, toyota-prius, box-truck, waymo-self-driving-car

Most vehicles have a material named "Accent" which is usually the paint or stripes. This is so that a managed street (future: group) can quickly apply colors to all "Accent" materials in a scene.

Added a tutorial for using Blender to add vertex colors, compressing, and verifying the output. It is linked from the README.

Notes:
Why does the file size change so dramatically for some assets?

From my understanding, adding any vertex group adds a new field to all vertices indicating which group they are a part of. This increases the file size absolutely, since there is a new group ID field. However, after compression it may be smaller than the original due to some optimizations that are used, since many vertices no longer need the uv coordinates.