Replace unit system constants with an Enum

[bachya]

Describe what the PR does:

This allows us the ability to type things more strictly.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5405382) 100.00% compared to head (84ea6a9) 100.00%.

❗ Current head 84ea6a9 differs from pull request most recent head f0d08db. Consider uploading reports for the commit f0d08db to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##               dev      #848   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           31        31           
  Lines         1763      1764    +1     
=========================================
+ Hits          1763      1764    +1     

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