Improvement: Deactivate cmesh load and save for now

[spenke91]

Closes #2265.

Describe your changes here:

We decided not to add a test to cmesh saving and loading, because they are heavily outdated anyway. Instead, we deactivate the code for now and throw an error message if it is called.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.59%. Comparing base (ad12034) to head (1f6b502).

Files with missing lines	Patch %	Lines
...8_cmesh_io/deprecated/deprecated_t8_cmesh_save.cxx	0.00%	6 Missing ⚠️
...esh_io/deprecated/deprecated_t8_cmesh_triangle.cxx	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2279      +/-   ##
==========================================
+ Coverage   79.42%   82.59%   +3.16%     
==========================================
  Files         115      115              
  Lines       19285    18545     -740     
==========================================
  Hits        15318    15318              
+ Misses       3967     3227     -740     

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[lenaploetzke]

In the remaining code, we prefer [[maybe_unused]], is there a reason why you decided to not use this?

[spenke91]

Yes, good point, I mixed something up here: Using [[maybe_unused]] in the source only, but not in the header (where we can't use it in this case, because it's a .h header) used to cause problems with the old doxygen version, but that is no longer an issue :-)

[lenaploetzke]

The example IO/cmesh/t8_cmesh_load_save.cxx is relying on this function, maybe you can add a warning or even leave this out via cmake or also with #if 0? There are also other examples in IO that maybe should get a comment at least

[lenaploetzke]

I am not a very big fan of the #if 0 and would prefer to somehow deactivate whole files via cmake but i understand the idea of printing error messages so maybe this is the way to go here. The functionality is well documented.