[ENH] Enable num_threads for antsAffineInitializer

[oesteban]

This PR just enables the general control for number of threads for
this tool. I just learned that it works differently depending on it
ANTsX/ANTs#444

[codecov-io]

Codecov Report

Merging #2010 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2010      +/-   ##
==========================================
+ Coverage   72.18%   72.19%   +<.01%     
==========================================
  Files        1132     1132              
  Lines       57008    57008              
  Branches     8162     8162              
==========================================
+ Hits        41152    41157       +5     
+ Misses      14572    14567       -5     
  Partials     1284     1284

Flag	Coverage Δ
#smoketests	72.19% <100%> (ø)	⬆️
#unittests	69.78% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
...terfaces/ants/tests/test_auto_AffineInitializer.py	85.71% <ø> (ø)	⬆️
nipype/interfaces/ants/utils.py	85.71% <100%> (ø)	⬆️
nipype/interfaces/utility/wrappers.py	86.36% <0%> (+4.54%)	⬆️

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[satra]

@oesteban - this likely needs a make specs update.

[oesteban]

You got it. Felt like a newbie when I read your comment :P

[satra]

@oesteban - we will address this before 1.0. better testing is coming to nipype (https://github.com/kaczmarj/neurodocker).

[oesteban]

Rerun the failing builds of Travis (due to apt's hiccups). This is ready to merge.