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 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (8185fb4) to head (6d32a6f).
⚠️ Report is 32 commits behind head on master.

Files with missing lines	Patch %	Lines
nitransforms/io/itk.py	0.00%	19 Missing ⚠️

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HEAD has 5 uploads less than BASE

Flag	BASE (8185fb4)	HEAD (6d32a6f)
	5	1
unittests	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #266       +/-   ##
==========================================
- Coverage   94.90%   0.00%   -94.91%     
==========================================
  Files          16      16               
  Lines        1982    1991        +9     
  Branches      265       0      -265     
==========================================
- Hits         1881       0     -1881     
- Misses         78    1991     +1913     
+ Partials       23       0       -23     

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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:

[oesteban]

The core of this PR has been split into #271, #272, #274, and #277. Closing to unblock the way for #264 (or any better implementation to address #171 (and indirectly #200).

[oesteban]

Keeping the branch for provenance purposes and for reference.