Improvement: Make FORTRAN interface work again and add tests for all its functions

[spenke91]

Closes #1816
Closes #1811

Describe your changes here:

For improved readability, this PR replaces #1815. It improves the FORTRAN interface by

• making it compile and work again with the current t8code version, fixing various little bugs;
• adding / extending the Fortran test cases such that all functions of the fortran interface are covered;
• properly adding the fortran files to the code coverage report.

All these boxes must be checked by the AUTHOR before requesting review:

• The PR is small enough to be reviewed easily. If not, consider splitting up the changes in multiple PRs.
• The title starts with one of the following prefixes: Documentation:, Bugfix:, Feature:, Improvement: or Other:.
• If the PR is related to an issue, make sure to link it.
• The author made sure that, as a reviewer, he/she would check all boxes below.

All these boxes must be checked by the REVIEWERS before merging the pull request:

As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

General

• The reviewer executed the new code features at least once and checked the results manually.
• The code follows the t8code coding guidelines.
• New source/header files are properly added to the CMake files.
• The code is well documented. In particular, all function declarations, structs/classes and their members have a proper doxygen documentation. Make sure to add a file documentation for each file!
• All new algorithms and data structures are sufficiently optimal in terms of memory and runtime (If this should be merged, but there is still potential for optimization, create a new issue).

Tests

• The code is covered in an existing or new test case using Google Test.
• The code coverage of the project (reported in the CI) should not decrease. If coverage is decreased, make sure that this is reasonable and acceptable.
• Valgrind doesn't find any bugs in the new code. This script can be used to check for errors; see also this wiki article.

If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

• Should this use case be added to the github action?
• If not, does the specific use case compile and all tests pass (check manually).

Scripts and Wiki

• If a new directory with source files is added, it must be covered by the script/find_all_source_files.scp to check the indentation of these files.
• If this PR introduces a new feature, it must be covered in an example or tutorial and a Wiki article.

License

• The author added a BSD statement to doc/ (or already has one).

[Davknapp]

Why did you delete this function?

[Davknapp]

What happens if MPI is disabled and this test is called?

[Davknapp]

Can you elaborate a little bit more (e.g. it create a cmesh and writes it....)

[Davknapp]

Suggested change

	!! Copyright (C) 2024 the developers
	!! Copyright (C) 2026 the developers

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.31%. Comparing base (b70bba4) to head (6a2e02c).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2177      +/-   ##
==========================================
+ Coverage   77.97%   78.31%   +0.33%     
==========================================
  Files         113      115       +2     
  Lines       19106    19119      +13     
==========================================
+ Hits        14898    14973      +75     
+ Misses       4208     4146      -62     

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