[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/PhysicsClient.h

@@ -34,6 +34,10 @@ public:
 
     virtual bool getJointInfo(int bodyUniqueId, int jointIndex, struct b3JointInfo& info) const = 0;
 
+    virtual int getNumUserConstraints() const = 0;
+    
+    virtual int getUserConstraintInfo(int constraintUniqueId, struct b3UserConstraint& info) const = 0;
+    
     virtual void setSharedMemoryKey(int key) = 0;
 
     virtual void uploadBulletFileToSharedMemory(const char* data, int len) = 0;