More visualization options: black default and gamma color scaling

[Timdnb]

For some figures I was working on I preferred to have black for (0,0) flow, so I added that option. Also added a gamma value to scale how fast the color changes, such that small flow values pop out better. For reference I also added a function to visualize the colorwheel (also helps understand the effect of the gamma value).

Thought it might be useful for someone else!

For reference, the black colorwheel with gamma=1.0

And the black colorwheel with gamma=0.5

Code used for these:

import flow_vis
import matplotlib.pyplot as plt

img = flow_vis.render_colorwheel(size=512, black=True, gamma=0.5, convert_to_bgr=False)

plt.imshow(img)
plt.axis('off')
plt.savefig('colorwheel.png', bbox_inches='tight', pad_inches=0)