Add compressed visualization

[gdalle]

• Change show_colors so that it includes the visualization of the compressed matrix. It now returns a tuple (A_img, B_img) or (A_img, Br_img, Bc_img).
• Demonstrate compressed visualization in the docs, on a column coloring and on a bidirectional coloring.
• Test the tail of the tuple (right now only A_img is tested for consistency).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (0246517) to head (23aee47).
Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           13        13           
  Lines         1257      1308   +51     
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+ Hits          1257      1308   +51     

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[amontoison]

Can you also support the neutral color 0?
Even if we don't do the postprocessing yet, it will be great to already support it.
We just need to use the color white if we encounter the neutral color or another simple trick.

[gdalle]

Can you also support the neutral color 0?

We could but at the moment there are lots of spots where we implicitly assume the colors start at 1. So I don't think it would make much sense to change this only in visualization utilities and not anywhere else. I have opened #158 to keep track of this more general overhaul.

In the meantime I'd rather merge this PR as-is to enable visualization for the blog post, and leave neutral colors for later.