Histogram Equalization (Normal and Adaptive) + Changes to VisionStack.py

[AlexanderWangY]

Added a histogram equalization layer which distributes pixel intensities (for a grayscale image) based on the histogram of the image pixels.

• Increases the contrast of an image slightly allowing for better corner detection especially in blurrier environments (like underwater)
• Balances the lighting of an image especially if lighting conditions differ

Also made some changes so that matplotlib's imshow correctly shows grayscale images in black and white!

[DaniParr]

Please reconfigure your version of vision stack and add your changes to the new version of vision stack. Then make the pull request again :)

[AlexanderWangY]

I'm a bit busy at the moment but I will reconfigure it as soon as I can if this PR is still wanted!