Unit test: segment_tractogram

[EmmaRenauld]

Yeah!

No changes, except that I moved the inversion of masks closer to the actual moment when we need to inverse them.
Added documentation.
Added a unit test for the function segment_tractogram_from_ROI

(** Unit tests for the library component: from 29.14% to 30.37% **)

[codecov]

Codecov Report

❌ Patch coverage is 77.27273% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.76%. Comparing base (ad967d5) to head (5646bb9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1290      +/-   ##
==========================================
+ Coverage   72.59%   72.76%   +0.16%     
==========================================
  Files         294      294              
  Lines       25339    25338       -1     
  Branches     3546     3545       -1     
==========================================
+ Hits        18395    18437      +42     
+ Misses       5448     5404      -44     
- Partials     1496     1497       +1     

Flag	Coverage Δ
smoketests	69.78% <59.09%> (+<0.01%)	⬆️
unittests	13.84% <45.45%> (+0.56%)	⬆️

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Components	Coverage Δ
Scripts	75.45% <100.00%> (+<0.01%)	⬆️
Library	69.55% <72.22%> (+0.36%)	⬆️

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