Improvement: Introduce github workflow using Clang and OpenMPI

[spenke91]

Closes #2056

Resulted from the discussion in #1528 - which can be closed afterwards.

Describe your changes here:

This PR introduces a new github workflow that tests t8code with Clang and OpenMPI. To not bloat the runtime, it only uses Debug mode, MPI ON, and no external libraries like VTK - which however, may easily be changed later if we decide we need that.
Note that the workflow uses an own docker image, that is created based on the new Dockerfile introduced in DLR-AMR/t8code-docker-images#3 (this PR is not merged yet, but the docker image has already been published by its CI workflow).

Don't worry, I will squash the commits before merging 😆

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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

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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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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 77.05%. Comparing base (e086633) to head (1e4593c).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2055   +/-   ##
=======================================
  Coverage   77.05%   77.05%           
=======================================
  Files         112      112           
  Lines       18959    18959           
=======================================
  Hits        14608    14608           
  Misses       4351     4351           

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[sandro-elsweijer]

Since this is only meant as a test for clang warnings and not for bugs etc., I would suggest to build t8code with the settings which compile the most of our codebase. Meaning that we should also enable VTK, OCC and NetCDF linkage and enable our Fortran API :)