Lay groundwork for dynamic unit selection

[bachya]

Describe what the PR does:

For #308, this PR lays the groundwork for dynamic unit selection beyond what we currently have.

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.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #324 (d5e070c) into dev (6e9eaec) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev      #324   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           29        29           
  Lines         1470      1516   +46     
=========================================
+ Hits          1470      1516   +46     

Impacted Files	Coverage Δ
ecowitt2mqtt/helpers/calculator/__init__.py	100.00% <100.00%> (ø)
ecowitt2mqtt/helpers/calculator/humidity.py	100.00% <100.00%> (ø)
ecowitt2mqtt/helpers/calculator/illuminance.py	100.00% <100.00%> (ø)
ecowitt2mqtt/helpers/calculator/lightning.py	100.00% <100.00%> (ø)
ecowitt2mqtt/helpers/calculator/precipitation.py	100.00% <100.00%> (ø)
ecowitt2mqtt/helpers/calculator/pressure.py	100.00% <100.00%> (ø)
ecowitt2mqtt/helpers/calculator/temperature.py	100.00% <100.00%> (ø)
ecowitt2mqtt/helpers/calculator/time.py	100.00% <100.00%> (ø)
ecowitt2mqtt/helpers/calculator/uv.py	100.00% <100.00%> (ø)
ecowitt2mqtt/helpers/calculator/wind.py	100.00% <100.00%> (ø)

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