[FIX] ensure that threshold passed to plotting functions are positive

[Remi-Gau]

• Relates to [BUG] avg_method and symmetric_cbar flags using plot_surf_stat_map working properly with plotly engine #4296 (comment)
• closes Passing negative values as threshold for plotting #4340

Changes proposed in this pull request:

As far as I can tell all plotting threshold are implicitly expected to be positive numbers but this is never checked or enforced.

• add a function to convert negative threshold to their absolute values if they are negative

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 92.14%. Comparing base (abb80ff) to head (8ab5efc).
Report is 37 commits behind head on main.

Files	Patch %	Lines
nilearn/plotting/img_plotting.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4338      +/-   ##
==========================================
+ Coverage   91.85%   92.14%   +0.28%     
==========================================
  Files         144      143       -1     
  Lines       16419    16509      +90     
  Branches     3434     3466      +32     
==========================================
+ Hits        15082    15212     +130     
+ Misses        792      751      -41     
- Partials      545      546       +1     

Flag	Coverage Δ
macos-latest_3.10_test_plotting	91.93% <94.11%> (?)
macos-latest_3.11_test_plotting	91.93% <94.11%> (+0.08%)	⬆️
macos-latest_3.12_test_plotting	91.93% <94.11%> (?)
macos-latest_3.8_test_plotting	91.89% <94.11%> (?)
macos-latest_3.9_test_plotting	91.90% <94.11%> (?)
ubuntu-latest_3.10_test_plotting	91.93% <94.11%> (+0.08%)	⬆️
ubuntu-latest_3.11_test_plotting	91.93% <94.11%> (?)
ubuntu-latest_3.12_test_plotting	91.93% <94.11%> (?)
ubuntu-latest_3.8_test_min	68.78% <5.88%> (?)
ubuntu-latest_3.8_test_plot_min	91.60% <70.58%> (?)
ubuntu-latest_3.8_test_plotting	91.89% <94.11%> (?)
ubuntu-latest_3.9_test_plotting	91.90% <94.11%> (?)
windows-latest_3.10_test_plotting	91.91% <94.11%> (?)
windows-latest_3.11_test_plotting	91.91% <94.11%> (?)
windows-latest_3.12_test_plotting	91.91% <94.11%> (?)
windows-latest_3.8_test_plotting	91.87% <94.11%> (?)
windows-latest_3.9_test_plotting	91.88% <94.11%> (?)

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[bthirion]

I'm wondering whether this is right. You could be in situations where you want to keep everything that's above -1, no ?
Why not leave the responsibility to the user ?