Implement Histogram of Gradients Backend

[krisharma]

Create an implementation of the HoG demo as outlined here: https://learnopencv.com/histogram-of-oriented-gradients/

We want to ensure that we store the following intermediate computations so we can show them to the students:

• g_x: the image resulting from sliding the horizontal Sobel filter across the original image (i.e. [-1, 0, 1])
• g_y: the image resulting from sliding the vertical Sobel filter across the original image (i.e. [-1, 0, 1]^T)
• Result of combining g_x and g_y to obtain the final vector direction + magnitudes
• Histogram of grouping the vectors into direction bins, the amount that goes into each bin is based on the magnitude of the vectors

Use OpenCV.js for easy computation of the Sobel operations.