Handle displacement field ordering in ITK HDF5 files

[oesteban]

Summary

• detect storage order of displacement fields saved in ITK HDF5 files
• add regression tests for C- and Fortran-order displacement arrays
• improve gradient-based detection of array ordering

Testing

• pytest -c /dev/null nitransforms/tests/test_io.py::test_itk_h5_field_order nitransforms/tests/test_io.py::test_itk_h5_field_order_fortran -q
• pytest -c /dev/null -q (fails: FileNotFoundError for missing test data)

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https://chatgpt.com/codex/tasks/task_e_687dea0dc7548330968169d3f4a9a574

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.13%. Comparing base (0dff054) to head (be5423a).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #260      +/-   ##
==========================================
- Coverage   95.22%   95.13%   -0.10%     
==========================================
  Files          16       16              
  Lines        1969     1994      +25     
  Branches      266      266              
==========================================
+ Hits         1875     1897      +22     
- Misses         75       77       +2     
- Partials       19       20       +1     

Flag	Coverage Δ
unittests	95.04% <ø> (ø)

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