Trixi.jl code cleanups + improvements (thanks Hendrik!)

[slayoo]

CC @ranocha

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.93%. Comparing base (67360ef) to head (187df3e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #604   +/-   ##
=======================================
  Coverage   92.93%   92.93%           
=======================================
  Files          40       40           
  Lines        1754     1754           
=======================================
  Hits         1630     1630           
  Misses        124      124           

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[github-actions]

Stale pull request message

[slayoo]

Thanks @ranocha
The one failing CI job is a windows timeout in a code that is genuinely unrelated to this PR, hence merging, and will look into it in another PR