Internally support voxel-wise confounds

[tsalo]

Closes #1243. This doesn't affect the command-line interface or ingression of confounds. It simply prepares for a time when we'll be able to ingress voxel-wise confounds.

Changes proposed in this pull request

• Allow a list of voxel-wise confounds in denoise_with_nilearn.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.10%. Comparing base (84ee283) to head (6b0e6f9).
Report is 53 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1244      +/-   ##
==========================================
+ Coverage   84.04%   84.10%   +0.06%     
==========================================
  Files          59       59              
  Lines        7182     7210      +28     
  Branches     1048     1064      +16     
==========================================
+ Hits         6036     6064      +28     
  Misses        882      882              
  Partials      264      264              

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