Add disable-n4 option

[tsalo]

Closes none, but helps with #578.

Changes proposed in this pull request

• Add --disable-n4 option to skip N4 bias correction step in the ASL brain mask creation workflow.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.82759% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.41%. Comparing base (9fb8536) to head (1a1ce58).

Files with missing lines	Patch %	Lines
aslprep/workflows/asl/reference.py	94.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #583      +/-   ##
==========================================
+ Coverage   76.19%   76.41%   +0.21%     
==========================================
  Files          38       38              
  Lines        4382     4435      +53     
==========================================
+ Hits         3339     3389      +50     
- Misses       1043     1046       +3     

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