Type stability of functions from examples tree_2d_dgsem

[huiyuxie]

Continue #2133.

[github-actions]

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Purpose and scope

• The PR has a single goal that is clear from the PR title and/or description.
• All code changes represent a single set of modifications that logically belong together.
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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.
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Documentation

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Testing

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Performance

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Verification

• The correctness of the code was verified using appropriate tests.
• If new equations/methods are added, a convergence test has been run and the results
  are posted in the PR.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.36%. Comparing base (7f6a860) to head (20f66c0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2145   +/-   ##
=======================================
  Coverage   96.36%   96.36%           
=======================================
  Files         480      480           
  Lines       38069    38073    +4     
=======================================
+ Hits        36684    36688    +4     
  Misses       1385     1385           

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

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