[VIZ] Add support for peak coloring

[AlexVCaron]

For now, the visualization module only supports coloring the peaks by orientation. It is fine for the current use. We could however easily add uniform coloring or, even more interesting, scalar coloring, e.g. to color using the peaks' fractions.

• Investigate coloring options in fury
• Modify viz.backends.fury and viz.slice to add the functionalities
• Add arguments in io.utils.add_peaks_screenshot_args
• Bind in the relevant scripts