Bugfix: Curved geometries

[sandro-elsweijer]

Closes #2074

Describe your changes here:
I am so sorry for the diff count. But a huge part of this PR is documentation.
Fixes a bug in our curved mesh algorithms in which in some edge cases geometries were either not linked to tree faces or they were linked with wrong parameters. This can be observed by executing the curved mesh tutorial for tetrahedra on main and on this branch.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.95%. Comparing base (d7259da) to head (0b7c511).
⚠️ Report is 65 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2073   +/-   ##
=======================================
  Coverage   77.95%   77.95%           
=======================================
  Files         113      113           
  Lines       19087    19087           
=======================================
  Hits        14880    14880           
  Misses       4207     4207           

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[lenaploetzke]

First part of review

[lenaploetzke]

Please also merge main

[lenaploetzke]

I think it is fine now, thanks for the work!