Use default case for Unit methods

[simon-lgb]

These long lists of all units are difficult to read and frequently cause merge conflicts with some custom units we have in our fork.

While using a default case can generally lead to unhandled enum constants when refactoring, it seems appropriate in these instances where this clearly should be the default behavior for new units.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #3753      +/-   ##
=============================================
- Coverage      60.61%   60.57%   -0.04%     
  Complexity       105      105              
=============================================
  Files           3356     3356              
  Lines         146350   146350              
  Branches       10852    10852              
=============================================
- Hits           88701    88637      -64     
- Misses         54313    54383      +70     
+ Partials        3336     3330       -6     

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