[pybullet] getNumConstraints, getConstraintInfo APIs.

[pybullet] updated pybullet_quickstartguide.pdf
Fail clearly (assert, return BT_INFINITY) if link index is out of range for btMultiBody methods localPosToWorld,worldPosToLocal,localDirToWorld,worldDirToLocal.
pybullet getConstraintInfo
Fix warnings due to Mac OSX 10.12 upgrade (with backward compatibility)

examples/SharedMemory/PhysicsLoopBack.h

@@ -48,6 +48,10 @@ public:
 
     virtual bool getJointInfo(int bodyIndex, int jointIndex, struct b3JointInfo& info) const;
 
+    virtual int getNumUserConstraints() const;
+    
+    virtual int getUserConstraintInfo(int constraintUniqueId, struct b3UserConstraint&info) const;
+    
 	///todo: move this out of the
     virtual void setSharedMemoryKey(int key);