ENH: Add X5 support of nonlinear transforms

[oesteban]

Extends support of nonlinear transforms to export/import X5 representation. New unit tests exercise the new I/O.

Cross-references: #241.

[codecov]

Codecov Report

Attention: Patch coverage is 96.72131% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.09%. Comparing base (4662d29) to head (27e95c7).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
nitransforms/nonlinear.py	96.49%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
+ Coverage   95.02%   95.09%   +0.07%     
==========================================
  Files          16       16              
  Lines        1869     1918      +49     
  Branches      241      251      +10     
==========================================
+ Hits         1776     1824      +48     
+ Misses         77       75       -2     
- Partials       16       19       +3     

Flag	Coverage Δ
unittests	94.91% <96.72%> (?)

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