ENH: Fix GP estimation error analysis script

[jhlegarreta]

• ENH: Fix return value type annotation in GP error analysis script func
• ENH: Use a boolean to tell whether to serialize the pandas df index
• BUG: Provide missing position arg to local cross validation function
• ENH: Add type annotation for variable in GP estimation error analysis

[jhlegarreta]

Marking as draft:

• This issue is still not solved:
  https://github.com/nipreps/nifreeze/actions/runs/12437972140/job/34728973936#step:8:110

  scripts/dwi_gp_estimation_error_analysis.py:77: error:
  Argument "cv" to "cross_val_score" has incompatible type "RepeatedKFold";
  expected "int | BaseCrossValidator | Iterable[Any] | BaseShuffleSplit | None"  [arg-type]
  

  and related to that, this is probably not what was planned when using RepeatedKFold:
  https://github.com/nipreps/nifreeze/pull/30/files#diff-e16a8632d36e294b24752d0ef9e6ea37a63e390a26045e188d8d5d4d0a9dc8ceR206

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.60%. Comparing base (0dadde7) to head (a19c66e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #30   +/-   ##
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  Coverage   67.60%   67.60%           
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  Files          20       20           
  Lines         991      991           
  Branches      131      131           
=======================================
  Hits          670      670           
  Misses        267      267           
  Partials       54       54           

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[jhlegarreta]

Most of these changes were addressed in PR #28. The only missing part is that the complaint in #30 (comment) seems legitimate, so ignoring the warning https://github.com/nipreps/nifreeze/pull/28/files#diff-e16a8632d36e294b24752d0ef9e6ea37a63e390a26045e188d8d5d4d0a9dc8ceR83 may not be the best solution. We are likely to revamp this script/the scripts as we make progress, so we will revisit this at due time. Keeping it open as a reminder.