FIX: Delay access of nibabel.trackvis until actually needed

[effigies]

Closes #3486.

[codecov]

Codecov Report

Merging #3488 (9f8e0b3) into master (3e7e613) will increase coverage by 16.73%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master    #3488       +/-   ##
===========================================
+ Coverage   48.52%   65.25%   +16.73%     
===========================================
  Files         309      309               
  Lines       40848    40846        -2     
  Branches     5378     5378               
===========================================
+ Hits        19820    26655     +6835     
+ Misses      20897    13117     -7780     
- Partials      131     1074      +943     

Flag	Coverage Δ
unittests	65.03% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nipype/interfaces/mrtrix/convert.py	15.73% <0.00%> (+14.22%)	⬆️
nipype/algorithms/mesh.py	29.70% <0.00%> (+0.49%)	⬆️
nipype/interfaces/camino/convert.py	71.92% <0.00%> (+0.76%)	⬆️
nipype/interfaces/dtitk/utils.py	92.66% <0.00%> (+0.91%)	⬆️
nipype/interfaces/cmtk/nbs.py	32.96% <0.00%> (+1.09%)	⬆️
nipype/pipeline/plugins/sge.py	18.10% <0.00%> (+1.29%)	⬆️
nipype/interfaces/freesurfer/petsurfer.py	77.45% <0.00%> (+1.73%)	⬆️
nipype/interfaces/mrtrix3/tracking.py	89.47% <0.00%> (+1.75%)	⬆️
nipype/interfaces/cmtk/cmtk.py	17.89% <0.00%> (+1.78%)	⬆️
nipype/interfaces/ants/visualization.py	87.75% <0.00%> (+2.04%)	⬆️
... and 110 more

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