add snippet aliquot_sum, is_triangle, _mode

[hetzz]

Added three snippets (aliquot_sum, is_triangle, _mode)

Description

aliquot_sum: returns the sum of the proper divisors of the number is_triangle: returns True/ False depending on the angles provided _mode: returns the mode of the list

What does your PR belong to?

• [ ] Website
• [X] Snippets
• [ ] General / Things regarding the repository (like CI Integration)

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] Enhancement (non-breaking improvement of a snippet)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [X] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have checked that the changes are working properly
• [X] I have checked that there isn't any PR doing the same
• [X] I have read the CONTRIBUTING document.