Fix checkstyle severity level mappings

[jzandbergen]

This PR addresses #1225.

It maps the the following Pint Severity levels to their appropriate Checkstyle
counterpart:

Pint severity	Checkstyle severity
Information	info
Warning	warning
Bug	error
Fatal	error
Unknown or not yet invented	error

The reasoning for mapping an "Unknown or not yet invented" pint severity to
checkstyle error rather be notified of a unimportant message than to miss
an important message.

Note: I've took the liberty to change the Checkstyle severities to lowercase as
it is specified in the checkstyle severity types

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.56%. Comparing base (6137d37) to head (62c6bce).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1702   +/-   ##
=======================================
  Coverage   99.56%   99.56%           
=======================================
  Files         109      109           
  Lines       13203    13215   +12     
=======================================
+ Hits        13146    13158   +12     
  Misses         32       32           
  Partials       25       25           

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[jzandbergen]

🎉 Thanks a lot!