Add function for splitting units and add a test

[kratman]

Description

Small patch to handle exponents in units. There is probably more that can be done to improve the value/unit parsing, but this fixes the bug for now.

Fixes #4608

Type of change

• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python -m pytest (or $ nox -s tests)
• The documentation builds: $ python -m pytest --doctest-plus src (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ nox -s quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[kratman]

Adding "eE" is what patches the bug, the function was split to make it easier to test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.21%. Comparing base (72c23ea) to head (6fb28bf).
Report is 8 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4641   +/-   ##
========================================
  Coverage    99.21%   99.21%           
========================================
  Files          302      302           
  Lines        22858    22866    +8     
========================================
+ Hits         22679    22687    +8     
  Misses         179      179           

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