ENH: Use the hot colormap for PET co-registration reports

[mnoergaard]

This PR addresses issue #350, by adding support for display PET images in PET-to-anatomical co-registration reports using Matplotlib’s hot colormap while keeping anatomical images grayscale. This improves the visibility of PET uptake patterns without changing the existing hover interaction.

Tests were added to verify the colormap configuration for both the standard co-registration and reference-mask reports.

[CPernet]

hot is no good, plasma a bit better -- but you can always linearize luminance eg https://github.com/CPernet/brain_colours/blob/master/code/software_linearization/standard_colormaps_comparison.png

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.49%. Comparing base (221c157) to head (ac8876a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #351   +/-   ##
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  Coverage   87.48%   87.49%           
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  Files          89       89           
  Lines       10794    10801    +7     
  Branches     1160     1160           
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+ Hits         9443     9450    +7     
  Misses       1122     1122           
  Partials      229      229           

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