ENH: X5 read/write support of TransformChain

[oesteban]

Summary

• Facilitates the reading of X5 transforms by adding a from_x5() function in the nitransforms.linear and nitransforms.nonlinear modules.
• Reading and writing of transform chains in X5 files, ensuring X5 files do not contain duplicated transforms.
• Ensures all individual transforms implement __eq__().

100% coverage of the patch.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.24%. Comparing base (fc6fbbd) to head (73f3db1).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
+ Coverage   95.09%   95.24%   +0.14%     
==========================================
  Files          16       16              
  Lines        1918     1976      +58     
  Branches      251      266      +15     
==========================================
+ Hits         1824     1882      +58     
  Misses         75       75              
  Partials       19       19              

Flag	Coverage Δ
unittests	95.06% <100.00%> (+0.14%)	⬆️

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