Add create surface from labels

[arnaudbore]

Quick description

First attempt to bring some of the nice scripts from Etienne to scilpy. I choose to reduce the scope of the original script to fit scilpy requirements I kept the expected behavior of the script used in nf-set.

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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)

Use scilpy atlas test data ...

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

[pep8speaks]

Hello @arnaudbore, Thank you for updating !

• In the file scripts/scil_surface_from_labels.py:

Line 137:80: E501 line too long (86 > 79 characters)