Fix issues related to RAP tracking

[anaislanthier]

Quick description

Added/Improved warnings related to RAP tracking.
Fixed an issue where step size did not change as intended during RAP tracking.
Replaced min_nbr_points and max_nbr_points by min_length and max_length in the Tracker class.
Merged Propagator on Sphere and ODF Propagator into one single class.

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Type of change

Check the relevant options.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Provide data, screenshots, command line to test (if relevant)

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Checklist

• My code follows the style guidelines of this project (run autopep8)
• I added relevant citations to scripts, modules and functions docstrings and descriptions
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I moved all functions from the script file (except the argparser and main) to scilpy modules
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.57%. Comparing base (aa4b761) to head (f34b97a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1364      +/-   ##
==========================================
- Coverage   72.60%   72.57%   -0.04%     
==========================================
  Files         301      301              
  Lines       26286    26279       -7     
  Branches     3700     3702       +2     
==========================================
- Hits        19086    19072      -14     
- Misses       5650     5654       +4     
- Partials     1550     1553       +3     

Flag	Coverage Δ
smoketests	69.73% <62.50%> (-0.04%)	⬇️
unittests	14.22% <0.00%> (+<0.01%)	⬆️

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

Components	Coverage Δ
Scripts	75.28% <21.42%> (-0.07%)	⬇️
Library	69.32% <84.61%> (-0.01%)	⬇️

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[CHrlS98]

Looks good, here are a few comments. Can you share some example data for testing?

[CHrlS98]

Tested with fibercup data, good to go @arnaudbore