Add tests for cells sub-module

[K-Meech]

Description

What is this PR

• Bug fix
• Addition of a new feature
• 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 #57
Adds docstrings for #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:

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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[alessandrofelder]

Looking good ✨
Some tiny comments - a lot of them are different instances of the same suggestion!

[K-Meech]

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