ENH: Collapse linear and nonlinear transforms chains

[oesteban]

Very undertested, but there is a new test that uses a "collapsed" transform from an ITK's .h5 file with one affine and one nonlinear (and it works).

BSpline transforms are not currently supported.

Related: #167, #169. Resolves #89.

[codecov[bot]]

Codecov Report

Patch coverage: 66.66% and project coverage change: -0.17 :warning:

Comparison is base (54ad1ea) 98.59% compared to head (fbc9228) 98.42%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
- Coverage   98.59%   98.42%   -0.17%     
==========================================
  Files          13       13              
  Lines        1279     1273       -6     
  Branches      184      183       -1     
==========================================
- Hits         1261     1253       -8     
- Misses         10       11       +1     
- Partials        8        9       +1     

Flag	Coverage Δ
travis	96.77% <66.66%> (-0.18%)	:arrow_down:
unittests	98.37% <66.66%> (-0.17%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nitransforms/linear.py	95.55% <33.33%> (-1.57%)	:arrow_down:
nitransforms/manip.py	100.00% <100.00%> (ø)

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[oesteban]

I will be resuscitating this one over this week. Thanks for your patience!

[oesteban]

@mattcieslak also this (I'm remembering as I go) :D