Explicitly check for -p none and --custom-confounds

[tsalo]

Closes none, but addresses an issue reported by @juliegrier. Basically, it's currently unclear how XCP-D works if users use -p none (no denoising) and --custom-confounds (external regressors to be used for denoising). Logically, it makes more sense to automatically change -p to custom in that scenario than to ignore --custom-confounds.

Changes proposed in this pull request

• If nuisance regressors are set to none, but custom confounds are provided, then issue a warning and change the nuisance regressor option to custom.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.29%. Comparing base (14de282) to head (59754c5).
Report is 53 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1222   +/-   ##
=======================================
  Coverage   84.28%   84.29%           
=======================================
  Files          54       54           
  Lines        7064     7067    +3     
  Branches     1027     1028    +1     
=======================================
+ Hits         5954     5957    +3     
  Misses        850      850           
  Partials      260      260           

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