Add tests for cells sub-module

[K-Meech]

Description

What is this PR

• [ ] Bug fix
• [ ] Addition of a new feature
• [X] Other

Why is this PR needed? The coverage of the cells sub-module is currently only 70%, and there are very few docstrings explaining its functionality.

What does this PR do?

This PR expands the tests and docstrings, bringing the cells sub-module coverage up to 90%.

References

Closes https://github.com/brainglobe/brainglobe-utils/issues/57 Adds docstrings for https://github.com/brainglobe/brainglobe-utils/issues/19

How has this PR been tested?

Tests have been updated, and all pass locally.

Is this a breaking change?

No

Does this PR require an update to the documentation?

No

Checklist:

• [X] The code has been tested locally
• [X] Tests have been added to cover all new functionality (unit & integration)
• [X] The documentation has been updated to reflect any changes
• [X] The code has been formatted with pre-commit

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.56%. Comparing base (264f849) to head (e5e58e5). Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #61 +/- ## ========================================== + Coverage 84.07% 86.56% +2.49% ========================================== Files 35 35 Lines 1331 1355 +24 ========================================== + Hits 1119 1173 +54 + Misses 212 182 -30 ```

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[K-Meech]

Thanks @alessandrofelder ! I've added all of your suggestions now - feel free to merge if you're happy with it.