FIX: Make ants.LaplacianThickness output_image a string, not file

[effigies]

Summary

Fixes #3390.

[codecov]

Codecov Report

Merging #3393 (93d6c3b) into master (071e575) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3393   +/-   ##
=======================================
  Coverage   65.21%   65.21%           
=======================================
  Files         307      307           
  Lines       40413    40413           
  Branches     5344     5344           
=======================================
  Hits        26356    26356           
  Misses      12982    12982           
  Partials     1075     1075           

Flag	Coverage Δ
unittests	64.94% <100.00%> (ø)

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Impacted Files	Coverage Δ
nipype/interfaces/ants/segmentation.py	76.51% <100.00%> (ø)

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