New script: Compute endpoints density map

[EmmaRenauld]

Quick description

I needed this small utilitary script. We already had:

• scil_tractogram_seed_density_map: Not the same. A seed can be in the middle of the streamline.
• scil_tractogram_compute_density_map: All the points. Could also be an option.
• scil_bundle_compute_endpoints_density_map: Outputs two maps: head and tail.

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Type of change

Check the relevant options.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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

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

[codecov[bot]]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (9412961) 68.12% compared to head (0df668d) 68.16%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #913 +/- ## ========================================== + Coverage 68.12% 68.16% +0.04% ========================================== Files 386 387 +1 Lines 20678 20722 +44 Branches 3160 3165 +5 ========================================== + Hits 14087 14126 +39 - Misses 5326 5328 +2 - Partials 1265 1268 +3 ``` | [Components](https://app.codecov.io/gh/scilus/scilpy/pull/913/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scilus) | Coverage Δ | | |---|---|---| | [Scripts](https://app.codecov.io/gh/scilus/scilpy/pull/913/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scilus) | `70.74% <88.63%> (+0.06%)` | :arrow_up: | | [Library](https://app.codecov.io/gh/scilus/scilpy/pull/913/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scilus) | `63.76% <ø> (ø)` | |