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

src/BulletDynamics/Featherstone/btMultiBodyConstraint.h

@@ -185,7 +185,9 @@ public:
 
 	virtual void setGearRatio(btScalar ratio) {}
 	virtual void setGearAuxLink(int gearAuxLink) {}
-
+	virtual void setRelativePositionTarget(btScalar relPosTarget){}
+	virtual void setErp(btScalar erp){}
+	
 	
 };