[ENH] ants.ApplyTransform - Accept 'identity' keyword

[oesteban]

Closes #2126

[codecov-io]

Codecov Report

Merging #2239 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2239      +/-   ##
==========================================
+ Coverage   72.31%   72.32%   +<.01%     
==========================================
  Files        1182     1182              
  Lines       58942    58944       +2     
  Branches     8490     8491       +1     
==========================================
+ Hits        42625    42632       +7     
+ Misses      14933    14929       -4     
+ Partials     1384     1383       -1

Flag	Coverage Δ
#smoketests	72.32% <100%> (ø)	⬆️
#unittests	69.89% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
nipype/interfaces/ants/resampling.py	80.45% <100%> (+1.97%)	⬆️
nipype/pipeline/plugins/multiproc.py	78.16% <0%> (+1.4%)	⬆️

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