ENH: Add MRTrix3 interfaces (pared-down gh-3426)

[effigies]

Replaces #3426.

Fixes #3400.
Fixes #3402.

Closes #3401.
Closes #3403.
Closes #3426.

[codecov]

Codecov Report

Merging #3445 (af8d36c) into master (e31573b) will increase coverage by 0.01%.
The diff coverage is 73.11%.

@@            Coverage Diff             @@
##           master    #3445      +/-   ##
==========================================
+ Coverage   65.21%   65.23%   +0.01%     
==========================================
  Files         308      308              
  Lines       40530    40619      +89     
  Branches     5353     5359       +6     
==========================================
+ Hits        26433    26497      +64     
- Misses      13021    13045      +24     
- Partials     1076     1077       +1     

Flag	Coverage Δ
unittests	64.95% <73.11%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
nipype/interfaces/mrtrix3/base.py	59.01% <50.00%> (-0.64%)	⬇️
nipype/interfaces/mrtrix3/utils.py	80.06% <72.94%> (-2.95%)	⬇️
nipype/interfaces/mrtrix3/__init__.py	100.00% <100.00%> (ø)

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