Initialize p4est and t8code only if possible

[JoshuaLampert]

With trixi-framework/P4est.jl#93 and DLR-AMR/T8code.jl#40 it is now possible to load P4est.jl and T8code.jl if MPIPreferences are set, but no preferences for P4est.jl or T8code.jl, respectively. For Trixi.jl this means that using MPI with a custom MPI implementation is more convenient, when p4est and/or t8code is not needed, e.g., if one wants to run a simulation that uses a TreeMesh since no preferences for P4est.jl and T8code.jl need to be set. This PR also prepares #1706, which can be merged after this PR.

[github-actions]

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 ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (1635d31) 96.28% compared to head (1b02a77) 96.27%.

Files	Patch %	Lines
src/auxiliary/t8code.jl	69.23%	4 Missing ⚠️
src/auxiliary/p4est.jl	66.67%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1745      +/-   ##
==========================================
- Coverage   96.28%   96.27%   -0.01%     
==========================================
  Files         428      428              
  Lines       34635    34639       +4     
==========================================
+ Hits        33345    33347       +2     
- Misses       1290     1292       +2     

Flag	Coverage Δ
unittests	96.27% <68.42%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sloede]

Thanks! LGTM once @ranocha is happy as well

[ranocha]

Thanks a lot 👍