expose gear erp/relative position target to C-API/pybullet

finish much better C++ vrSyncPlugin, running in-the-loop with the physics at high frequency, see also vr_kuka_setup_vrSyncPlugin.py

examples/SharedMemory/SharedMemoryPublic.h

@@ -247,6 +247,8 @@ struct b3UserConstraint
     int m_userConstraintUniqueId;
 	double m_gearRatio;
 	int m_gearAuxLink;
+	double m_relativePositionTarget;
+	double m_erp;
 
 };