Feature: Very first version of the unstructured mesh interface

[lenaploetzke]

Describe your changes here:

Closes #1655 This PR implements the very basic structure of the unstructured mesh interface. It provides only the level variable. Other variables should be implemented in future PRs.

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 98.63014% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 76.60%. Comparing base (41a06b6) to head (5147546).

Files with missing lines	Patch %	Lines
api/t8_mesh_handle/mesh.hxx	96.15%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1829      +/-   ##
==========================================
+ Coverage   76.52%   76.60%   +0.08%     
==========================================
  Files         106      109       +3     
  Lines       18655    18728      +73     
==========================================
+ Hits        14275    14347      +72     
- Misses       4380     4381       +1     

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[sandro-elsweijer]

For me, the APIs of t8code are the C, Fortran, Python... APIs. So interfaces to different programming languages. The mesh handle is more like a part of the actual library and less a separate API. At least it is different than the language APIs. So I would either put it in src or create a whole new folder. For me at least it does not fit in the api folder

[lenaploetzke]

For me, the APIs of t8code are the C, Fortran, Python... APIs. So interfaces to different programming languages. The mesh handle is more like a part of the actual library and less a separate API. At least it is different than the language APIs. So I would either put it in src or create a whole new folder. For me at least it does not fit in the api folder

It does not feel right for me to put the handle in the src folder but i am fine with putting it in its own. The idea was that the mesh handle is an api for other software so i think it is not really wrong in the folder. What do you think @spenke91 ?