Type stability of self-defined functions from examples

[huiyuxie]

Same as title.

[github-actions]

Review checklist

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Purpose and scope

• The PR has a single goal that is clear from the PR title and/or description.
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Code quality

• The code can be understood easily.
• Newly introduced names for variables etc. are self-descriptive and consistent with existing naming conventions.
• There are no redundancies that can be removed by simple modularization/refactoring.
• There are no leftover debug statements or commented code sections.
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Documentation

• New functions and types are documented with a docstring or top-level comment.
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• Comments describe intent ("why?") and not just functionality ("what?").
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Testing

• The PR passes all tests.
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• New or modified tests run in less then 10 seconds.

Performance

• There are no type instabilities or memory allocations in performance-critical parts.
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Verification

• The correctness of the code was verified using appropriate tests.
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  are posted in the PR.

Created with ❤️ by the Trixi.jl community.

[huiyuxie]

See #2112 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.36%. Comparing base (a8967c6) to head (12a0f46).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2122   +/-   ##
=======================================
  Coverage   96.36%   96.36%           
=======================================
  Files         477      477           
  Lines       37752    37753    +1     
=======================================
+ Hits        36377    36378    +1     
  Misses       1375     1375           

Flag	Coverage Δ
unittests	96.36% <100.00%> (+<0.01%)	⬆️

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[ranocha]

Thanks