3.21
reduce stack usage (cause some crashes in low-stack tests) fix crashing bug in changeVisualShape add differential gear version of racecar (only 2 back wheels are powered)
data: add license files (mostly copied from the URDF files)
more fixes in pybullet/resources, moved enjoy/train/test in 'examples' folder.
allow auxilary link to be used for gear btMultiBodyGearConstraint.