Write unit tests

[Shi-Ho]

https://docs.pytest.org/en/8.2.x/ (easier to use than https://docs.python.org/3/library/unittest.html I think)

• Write tests first for function with the less dependencies (so it is easier to debug than the other way around)
• Define edge cases behaviour
• Split or refactor functions first if needed for tests to make sense
• Should be in a dedicated tests directory
• Will be implemented in a workflow in the future
• Have different test files per theme/module/usecase/TBD