Towards partitioned RHS's (1D): TreeMesh only

[DanielDoehring]

This adds more functions which can be used to realize multirate time integration through partitioned Runge-Kutta.

main branch:

julia> Trixi.calc_viscous_fluxes!
calc_viscous_fluxes! (generic function with 4 methods)

This branch:

julia> Trixi.calc_viscous_fluxes!
calc_viscous_fluxes! (generic function with 5 methods)

[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.
• All code changes represent a single set of modifications that logically belong together.
• No more than 500 lines of code are changed or there is no obvious way to split the PR into multiple PRs.

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.
• The code adheres to our conventions and style guide, and to the Julia guidelines.

Documentation

• New functions and types are documented with a docstring or top-level comment.
• Relevant publications are referenced in docstrings (see example for formatting).
• Inline comments are used to document longer or unusual code sections.
• Comments describe intent ("why?") and not just functionality ("what?").
• If the PR introduces a significant change or new feature, it is documented in NEWS.md with its PR number.

Testing

• The PR passes all tests.
• New or modified lines of code are covered by tests.
• New or modified tests run in less then 10 seconds.

Performance

• There are no type instabilities or memory allocations in performance-critical parts.
• If the PR intent is to improve performance, before/after time measurements are posted in the PR.

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.

Created with ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

❌ Patch coverage is 73.68421% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.94%. Comparing base (e6496e2) to head (f0fb7be).

Files with missing lines	Patch %	Lines
...emidiscretization/semidiscretization_hyperbolic.jl	0.00%	9 Missing ⚠️
src/solvers/dgsem_tree/dg_1d.jl	92.31%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2633      +/-   ##
==========================================
- Coverage   96.83%   94.94%   -1.89%     
==========================================
  Files         537      537              
  Lines       42820    42828       +8     
==========================================
- Hits        41464    40663     -801     
- Misses       1356     2165     +809     

Flag	Coverage Δ
unittests	94.94% <73.68%> (-1.89%)	⬇️

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