enh: facilitate displacementsfield transform loading

[mgxd]

adds load functionality for nonlinear transforms similar to linear.

Will need to be revised once BSplineFieldTransform is re-added

[pull-assistant]

Best reviewed: commit by commit

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     enh: facilitate displacementsfield transform loading

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[codecov-io]

Codecov Report

Merging #54 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   98.76%   98.77%   +0.01%     
==========================================
  Files          10       10              
  Lines         810      819       +9     
  Branches      106      107       +1     
==========================================
+ Hits          800      809       +9     
  Misses          6        6              
  Partials        4        4

Flag	Coverage Δ
#unittests	98.77% <100%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
nitransforms/nonlinear.py	100% <100%> (ø)	⬆️

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[oesteban]

Neat!