FIX: Ordering of ITK displacements fields when reading and writing

[oesteban]

Transposes data as suggested by @feilong when reading and writing ITK displacements fields.

Tests have been added to ensure round-trip consistency.

Related-to: #171.

[oesteban]

@feilong apologies for the noise of the force-pushes

[codecov]

Codecov Report

❌ Patch coverage is 0% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (5647caf) to head (eee5a4e).

Files with missing lines	Patch %	Lines
nitransforms/nonlinear.py	0.00%	19 Missing ⚠️
nitransforms/io/itk.py	0.00%	12 Missing ⚠️
nitransforms/resampling.py	0.00%	10 Missing ⚠️
nitransforms/base.py	0.00%	5 Missing ⚠️

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HEAD has 27 uploads less than BASE

Flag	BASE (5647caf)	HEAD (eee5a4e)
	27	1
unittests	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #266       +/-   ##
==========================================
- Coverage   95.09%   0.00%   -95.10%     
==========================================
  Files          16      16               
  Lines        1979    1984        +5     
  Branches      265       0      -265     
==========================================
- Hits         1882       0     -1882     
- Misses         77    1984     +1907     
+ Partials       20       0       -20     

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[feilong]

@oesteban No worries. Happy to see the issue fixed in nitransforms.

[oesteban]

It doesn't look this transposition is the right answer (pictures extracted within the test_densefield_map_vs_ants test in this PR, which uses @feilong's 01_ants_t1_to_mniComposite_DisplacementFieldTransform.nii.gz:

Displacements with antsApplyTransformsToPoints:

Displacements with nitransforms: