WIP: Add partial volume correction workflow to PETPrep

[mnoergaard]

The work branch introduces partial volume correction (PVC) workflow options. Documentation now clarifies that when PVC is enabled, the corrected images are aligned to anatomy and propagate through subsequent spatial normalization steps. The CLI initializes output spaces to include native T1w by default, and configuration now records the chosen PVC tool alongside method and PSF parameters.

PVC interfaces include updated label handling: Binarise4DSegmentation now preserves background labels, and CSVtoNifti expects the label list to contain background as the first entry. Workflow logic uses these corrected images as inputs for standard‑space derivatives. When PVC is enabled, the workflow forwards the method name to output datasinks so derivative files include a _pvc- entity.

Tests verify that the PVC method propagates to derivatives and that PVC‑corrected images feed into standard‑space workflows. Unit tests for segmentation focus on helper functions and node selection.

Major changes

1. PVC parameters are now stored in configuration and propagated through datasinks.
2. Standard-space resampling uses PVC outputs when available, improving derivative consistency.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 117 lines in your changes missing coverage. Please review.

Project coverage is 0.39%. Comparing base (8c27a7a) to head (6d3339f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
petprep/workflows/pet/tests/test_base.py	0.00%	44 Missing ⚠️
petprep/interfaces/pvc.py	0.00%	23 Missing ⚠️
petprep/workflows/pet/base.py	0.00%	21 Missing ⚠️
petprep/cli/parser.py	0.00%	12 Missing ⚠️
petprep/cli/tests/test_parser.py	0.00%	7 Missing ⚠️
petprep/workflows/pet/pvc.py	0.00%	4 Missing ⚠️
petprep/config.py	0.00%	2 Missing ⚠️
petprep/workflows/pet/outputs.py	0.00%	2 Missing ⚠️
petprep/workflows/pet/resampling.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main     #35      +/-   ##
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- Coverage   0.40%   0.39%   -0.02%     
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  Files         69      69              
  Lines       4893    5026     +133     
  Branches     533     540       +7     
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  Hits          20      20              
- Misses      4873    5006     +133     

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