[MAINT] change function privacy in _utils

[Remi-Gau]

• Relates to Improve testing of _utils.data_gen #2750 and Check for inconsistent naming of functions according to PEP8 convention #3319
• Closes make sure public / private functions in _utils to start without / with a leading underscore #3628

Changes proposed in this pull request:

• ensure that private functions to the module have leading underscore
• ensure that public functions to the module have no leading underscore

[github-actions]

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[codecov]

Codecov Report

Merging #4061 (493ca0e) into main (96755a4) will not change coverage.
The diff coverage is 97.47%.

@@           Coverage Diff           @@
##             main    #4061   +/-   ##
=======================================
  Coverage   91.60%   91.60%           
=======================================
  Files         143      143           
  Lines       16072    16072           
  Branches     3339     3339           
=======================================
  Hits        14722    14722           
  Misses        804      804           
  Partials      546      546           

Flag	Coverage Δ
macos-latest_3.10	?
macos-latest_3.11	?
macos-latest_3.12	91.49% <96.63%> (ø)
macos-latest_3.8	91.45% <96.63%> (ø)
macos-latest_3.9	?
ubuntu-latest_3.10	91.49% <96.63%> (ø)
ubuntu-latest_3.11	91.49% <96.63%> (ø)
ubuntu-latest_3.12	?
ubuntu-latest_3.8	91.45% <96.63%> (ø)
ubuntu-latest_3.9	?
windows-latest_3.10	91.43% <96.63%> (ø)
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windows-latest_3.8	91.39% <96.63%> (ø)
windows-latest_3.9	91.40% <96.63%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
nilearn/_utils/__init__.py	100.00% <100.00%> (ø)
nilearn/_utils/data_gen.py	95.52% <100.00%> (ø)
nilearn/_utils/fmriprep_confounds.py	100.00% <100.00%> (ø)
nilearn/_utils/helpers.py	82.75% <100.00%> (ø)
nilearn/_utils/logger.py	100.00% <100.00%> (ø)
nilearn/_utils/ndimage.py	88.88% <100.00%> (ø)
nilearn/_utils/niimg.py	97.11% <100.00%> (ø)
nilearn/_utils/niimg_conversions.py	90.47% <100.00%> (ø)
nilearn/_utils/param_validation.py	100.00% <100.00%> (ø)
nilearn/_utils/path_finding.py	100.00% <100.00%> (ø)
... and 27 more

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[bthirion]

LGTM overall.

[Remi-Gau]

apparently we get some errors in doc build following this one.