FIX: Improve handling of optional fields in LTA

[effigies]

Closes #64.

No test right now, but it does handle the file provided there.

[pull-assistant]

Best reviewed: commit by commit

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Optimal code review plan

     FIX: Allow optional fields to be missing in the middle of a file

     MNT: Require nibabel 3.0+

     TEST: Add simple regression test

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[codecov-io]

Codecov Report

Merging #65 into master will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   98.52%   98.63%   +0.10%     
==========================================
  Files          11       11              
  Lines         951      950       -1     
  Branches      130      129       -1     
==========================================
  Hits          937      937              
  Misses          7        7              
+ Partials        7        6       -1     

Flag	Coverage Δ
#unittests	98.63% <100.00%> (+0.10%)	⬆️

Impacted Files	Coverage Δ
nitransforms/io/lta.py	99.32% <100.00%> (+0.66%)	⬆️

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[oesteban]

Awesome, thanks!