Replace with opencv NMS

This version replaces the original hard-coded NMS with the opencv cv2.dnn.NMSBoxes method.
This simplifies the code and improve portability to other programing languages.
Performance-wise it is similar to potentially slightly faster.
The function signature is the same except the default scoreThreshold is now 0.5

Other changes:

• the method in MTM.NMS PointInRect and computeIoU have been removed (not used anymore for NMS)
• tutorials updated with timeit benchmarks