Improvement: Custom competence test for mesh interface

[lenaploetzke]

Closes #1896

Describe your changes here:
The unstructured mesh interface allows the use of custom competences. A simple test is added for this.
Moreover there was a problem with the access specifier such that the caches were not actually used (because private members of course are not inherited). I need the public dummy functions because protected members cannot be used for the functionality to check if a cached version exists in the competences.
Lastly, the test is now faster because the get_centroid function is called less.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.62%. Comparing base (0cd9d49) to head (0d538bf).
⚠️ Report is 10 commits behind head on 1655-define-structure-of-the-unstructured-mesh-class.

Additional details and impacted files

@@                                   Coverage Diff                                    @@
##           1655-define-structure-of-the-unstructured-mesh-class    #1895      +/-   ##
========================================================================================
- Coverage                                                 75.91%   75.62%   -0.30%     
========================================================================================
  Files                                                       105      106       +1     
  Lines                                                     19248    19387     +139     
========================================================================================
+ Hits                                                      14613    14662      +49     
- Misses                                                     4635     4725      +90     

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[sandro-elsweijer]

Looks almost good to me :)