remove tests related to lab 2 and lab 3
add todo list related to lab 4
add partial_derivatives and element_multiply functions
working on harris_corner_detector function
add new code to harris_corner_detector function
add cv2 support
large commit: mostly complete lab4

	convolution:
		now use numpy array instead of list
	add 2 functions related to lab4:
	 	corner_response
		corner_threshold