ENH: Add initial X5 support of transforms chains

[oesteban]

Summary

• allow loading .x5 files into TransformChain
• test TransformChain X5 loading
• document support in changelog

Testing

• pytest -q nitransforms/tests/test_x5.py::test_transformchain_from_x5

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https://chatgpt.com/codex/tasks/task_e_687bb72cab988330a5e9113198dce53e

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 95.03%. Comparing base (4662d29) to head (8e7abcc).

Files with missing lines	Patch %	Lines
nitransforms/nonlinear.py	69.23%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
+ Coverage   95.02%   95.03%   +0.01%     
==========================================
  Files          16       16              
  Lines        1869     1893      +24     
  Branches      241      246       +5     
==========================================
+ Hits         1776     1799      +23     
+ Misses         77       76       -1     
- Partials       16       18       +2     

Flag	Coverage Δ
unittests	94.84% <83.33%> (?)

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[oesteban]

Closing this one as it duplicates some work of #249, which is undesirable. Will work on a new PR at a later time.