FIX: mark .niml.dset as special extension in utils.filemanip

[mvdoc]

All AFNI commands work also on files with extension .niml.dset (for surfaces).

Changes proposed in this pull request

• consider *.niml.dset as special extension to correctly split filenames

[codecov-io]

Codecov Report

Merging #2495 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2495   +/-   ##
=======================================
  Coverage   66.86%   66.86%           
=======================================
  Files         328      328           
  Lines       42560    42560           
  Branches     5282     5282           
=======================================
  Hits        28457    28457           
  Misses      13405    13405           
  Partials      698      698

Flag	Coverage Δ
#smoketests	50.79% <100%> (ø)	⬆️
#unittests	64.21% <100%> (-0.04%)	⬇️

Impacted Files	Coverage Δ
nipype/utils/filemanip.py	76.12% <100%> (ø)	⬆️

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[oesteban]

LGTM