FIX: Refactor of LTA implementation

[oesteban]

This PR makes the LTA i/o implementation more consistent with the others, cleaning up some bugs, completing some missing aspects of the implementation (which help reduce the XFailed tests by 4).

Related: #40.

[codecov-commenter]

Codecov Report

Merging #145 (a5b9752) into master (b3b188e) will decrease coverage by 0.37%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
- Coverage   98.66%   98.28%   -0.38%     
==========================================
  Files          12       12              
  Lines        1122     1109      -13     
  Branches      172      173       +1     
==========================================
- Hits         1107     1090      -17     
- Misses          5       11       +6     
+ Partials       10        8       -2     

Flag	Coverage Δ
travis	98.28% <100.00%> (-0.38%)	:arrow_down:

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Impacted Files	Coverage Δ
nitransforms/io/__init__.py	100.00% <ø> (ø)
nitransforms/io/fsl.py	97.97% <ø> (+2.97%)	:arrow_up:
nitransforms/io/base.py	100.00% <100.00%> (ø)
nitransforms/io/lta.py	99.45% <100.00%> (+0.12%)	:arrow_up:
nitransforms/linear.py	97.10% <100.00%> (-0.56%)	:arrow_down:
nitransforms/io/afni.py	89.24% <0.00%> (-7.53%)	:arrow_down:

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[oesteban]

In the interest of making progress at other points and getting rid of the xfail marks asap, I'll go ahead and merge.