Add ExplicitImports.jl test

[JoshuaLampert]

This adds a test using ExplicitImports.jl to check whether we have unused explicit imports or any implicit imports. It also includes the necessary changes to make the test pass.

[github-actions[bot]]

Review checklist

This checklist is meant to assist creators of PRs (to let them know what reviewers will typically look for) and reviewers (to guide them in a structured review process). Items do not need to be checked explicitly for a PR to be eligible for merging.

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.

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 :heart: by the Trixi.jl community.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.48%. Comparing base (2dfde7f) to head (3bb2ffa).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1875      +/-   ##
==========================================
- Coverage   96.30%   89.48%   -6.82%     
==========================================
  Files         439      439              
  Lines       35749    35738      -11     
==========================================
- Hits        34428    31979    -2449     
- Misses       1321     3759    +2438     

Flag	Coverage Δ
unittests	89.48% <ø> (-6.82%)	:arrow_down:

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[JoshuaLampert]

Codecov seems to be down for some reason. And then there are the known t8code issues again.

[JoshuaLampert]

Closing and reopening this PR to trigger CI again (I don't have the rights to rerun the jobs without closing and opening).

[JoshuaLampert]

Codecov is down again...