Add default options to cbf, te_asl and dw_asl scripts

[Copilot]

This PR adds default parameter values to the cbf, te_asl, and dw_asl scripts, making them consistent with the existing generate_subtracted_asl_image script.

Changes Made

The following parameters are now optional with sensible defaults:

• PLD (Post Labeling Delay): [170.0, 270.0, 370.0, 520.0, 670.0, 1070.0, 1870.0]
• LD (Labeling Duration): [100.0, 100.0, 150.0, 150.0, 400.0, 800.0, 1800.0]
• TE (Time of Echos): [13.56, 67.82, 122.08, 176.33, 230.59, 284.84, 339.1, 393.36] (te_asl only)
• DW (Diffusion weights): [0, 50, 100, 250] (dw_asl only)

Before

# These commands would fail with "required arguments" errors
python -m asltk.scripts.cbf pcasl.nii m0.nii output/
python -m asltk.scripts.te_asl pcasl.nii m0.nii output/
python -m asltk.scripts.dw_asl pcasl.nii m0.nii output/

After

# These commands now work with default values
python -m asltk.scripts.cbf pcasl.nii m0.nii output/
python -m asltk.scripts.te_asl pcasl.nii m0.nii output/
python -m asltk.scripts.dw_asl pcasl.nii m0.nii output/

# Manual parameters still work for custom protocols
python -m asltk.scripts.cbf pcasl.nii m0.nii output/ --pld 100 200 --ld 50 60

Benefits

• Improved usability: Scripts can now be run without requiring domain expertise to specify timing parameters
• Consistency: All ASL scripts now have the same default parameter behavior
• Backward compatibility: Existing workflows with manual parameters continue to work unchanged
• Documentation: Help text now clearly indicates when default values are available

Testing

• ✅ All existing tests pass (214/214)
• ✅ Scripts work with default parameters
• ✅ Scripts maintain backward compatibility with manual parameters
• ✅ Help output correctly shows parameters as optional

Fixes #22.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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