const and immutable structs, non-breaking

[DanielDoehring]

Non-breaking stuff from #2640

[github-actions]

Review checklist

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Created with ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.86%. Comparing base (81b8662) to head (130817c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2659      +/-   ##
==========================================
- Coverage   96.86%   96.86%   -0.00%     
==========================================
  Files         546      546              
  Lines       43209    43204       -5     
==========================================
- Hits        41854    41849       -5     
  Misses       1355     1355              

Flag	Coverage Δ
unittests	96.86% <100.00%> (-<0.01%)	⬇️

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[JoshuaLampert]

Thanks! I just have one question.