Implement btMultiBodySphericalJointMotor, able to track a quaternion position target.

Expose this btMultiBodySphericalJointMotor through PyBullet.setJointMotorControlMultiDof
Expose PyBullet.getQuaternionSlerp
Improve PyBullet.setJointMotorControlMultiDof
Improve humanoidMotionCapture.py with slerp and using setJointMotorControlMultiDof
Expose  btMultiBody::spatialTransform
Fix btMultiBody::setupPlanar from DeepMimic codebase
Add support for multidof joints in btMultiBody::compTreeLinkVelocities, thanks to DeepMimic codebase @xbpeng

src/BulletDynamics/CMakeLists.txt

@@ -41,6 +41,7 @@ SET(BulletDynamics_SRCS
 	Featherstone/btMultiBodyMLCPConstraintSolver.cpp
 	Featherstone/btMultiBodyPoint2Point.cpp
 	Featherstone/btMultiBodySliderConstraint.cpp
+	Featherstone/btMultiBodySphericalJointMotor.cpp
 	MLCPSolvers/btDantzigLCP.cpp
 	MLCPSolvers/btMLCPSolver.cpp
 	MLCPSolvers/btLemkeAlgorithm.cpp