Added utils function for 0-transparent cmaps

[timtreis]

Note: the only existing utils function was using a different testing pattern, which I now aligned with the usual one. But therefore there's an additonal image in this PR

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.35%. Comparing base (91e5726) to head (9ebc9ad).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #302      +/-   ##
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+ Coverage   80.18%   80.35%   +0.17%     
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  Files          11       11              
  Lines        1736     1746      +10     
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+ Hits         1392     1403      +11     
+ Misses        344      343       -1     

Files	Coverage Δ
src/spatialdata_plot/pl/utils.py	70.21% <100.00%> (+0.44%)	⬆️

[LucaMarconato]

Amazing! Ready for review?

[timtreis]

Added the function @LucaMarconato @giovp. Probably best to add it to one of the notebooks to illustrate usage - which one do you think would be best?

[LucaMarconato]

Thanks! I am working on the Visium HD notebook right now, so I can add it there 😊

[melonora]

LGTM! Maybe one consideration is to add a parameter that automatically does this for the user e,g, zero_transparent, but this could be added in a different PR. Also this PR does make it a bit more difficult for view configurations as we are dealing with different alpha values. May have to think of serialization of the colormap values.

[timtreis]

Yeah, based on #242 we decided not to include it as an argument but like this. But I agree, eventually, we should probably just serialise the data. I think ggplot f.e. also does it like this when generating the plot