Enhancement: Clean up cmesh folder. New subfolders and better structure. Only moving files.

[holke]

Closes #1991

Describe your changes here:

Please note that indentation is taken for in #1987 since so many files needed indentation that it deserves its own PR.
So if all tests except indent succeed, we can merge this.

The cmesh header and source files were a mess. For example t8_cmesh.h and other files were directly in the src/ folder instead of being part of the src/t8_cmesh subfolder.

Additionally, the subfolder needed some clean up.

• Created new subfolders
• • src/t8_cmesh/t8_cmesh_internal for internal header and source files that should not be open to users.
• • src/t8_cmesh/t8_cmesh_io for input and output related cmesh files
• Moved src/t8_cmesh*.h and .c files into src/t8_cmesh to clean up src/ folder.
• Adapted all #include lines to the new file names
• Changed all cmesh include path that still used "" to <>

Here is a screenshot of the folder structure:

Before:

After:

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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.
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Tests

• The code is covered in an existing or new test case using Google Test.
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• 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):

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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).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.52%. Comparing base (ab79801) to head (dd64094).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1986   +/-   ##
=======================================
  Coverage   76.52%   76.52%           
=======================================
  Files         106      106           
  Lines       18656    18656           
=======================================
  Hits        14276    14276           
  Misses       4380     4380           

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