enable btGearConstraint, expose 'changeDynamics' for gearRatio, only works for maximalCoordinates rigid bodies.

See examples\pybullet\examples\mimicJointConstraint.py

examples/pybullet/examples/mimicJointConstraint.py

@@ -0,0 +1,16 @@
+#mimic joint currently only works for 'maximal coordinate' rigid bodies
+#one way to use it would be to attach maximal coordinate rigid bodies to multibody links using
+#fixed constraints
+
+import pybullet as p
+import time
+p.connect(p.GUI)
+wheelA = p.loadURDF("wheel.urdf",[0,0,0],useMaximalCoordinates=1)
+wheelB = p.loadURDF("wheel.urdf",[0,0,1],useMaximalCoordinates=1)
+c = p.createConstraint(wheelA,-1,wheelB,-1,jointType=p.JOINT_GEAR,jointAxis =[0,1,0],parentFramePosition=[0,0,0],childFramePosition=[0,0,0])
+p.changeConstraint(c,gearRatio=-0.1)
+p.setRealTimeSimulation(1)
+while(1):
+	time.sleep(0.01)
+p.removeConstraint(c)
+