ENH: Uber-refactor of code style, method names, etc.

[oesteban]

Includes a draft implementation of #22.

[codecov-io]

Codecov Report

Merging #24 into master will decrease coverage by 2.01%.
The diff coverage is 83.6%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
- Coverage   77.04%   75.03%   -2.02%     
==========================================
  Files          11       11              
  Lines         758      733      -25     
  Branches       92       95       +3     
==========================================
- Hits          584      550      -34     
- Misses        136      147      +11     
+ Partials       38       36       -2

Flag	Coverage Δ
#unittests	75.03% <83.6%> (-2.02%)	⬇️

Impacted Files	Coverage Δ
nitransforms/conftest.py	100% <ø> (ø)	⬆️
nitransforms/linear.py	63.49% <100%> (-6.57%)	⬇️
nitransforms/tests/test_transform.py	82.75% <100%> (-2.25%)	⬇️
nitransforms/tests/test_base.py	100% <100%> (ø)	⬆️
nitransforms/tests/test_io.py	100% <100%> (ø)	⬆️
nitransforms/nonlinear.py	39.43% <62.5%> (-4.78%)	⬇️
nitransforms/base.py	67.39% <80%> (-5.65%)	⬇️

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[mgxd]

nice, i think this makes the api cleaner and makes conversions from ras / vox spaces a bit clearer. looks good - only complaint is coverage is currently lower.