FIX: updates input spec to allow 4D images

[miykael]

ANTs' buildtemplateparallel.sh and N4BiasFieldCorrection allow 4D images as input files. Nipype restricted the inputs to be 2D or 3D.

[codecov-io]

Codecov Report

Merging #2151 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2151   +/-   ##
=======================================
  Coverage   72.25%   72.25%           
=======================================
  Files        1168     1168           
  Lines       58405    58405           
  Branches     8400     8400           
=======================================
  Hits        42201    42201           
  Misses      14870    14870           
  Partials     1334     1334

Flag	Coverage Δ
#smoketests	72.25% <100%> (ø)	⬆️
#unittests	69.91% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
nipype/interfaces/ants/segmentation.py	72.79% <100%> (ø)	⬆️
nipype/interfaces/ants/legacy.py	66.66% <100%> (ø)	⬆️

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