Feature: Add subelement scheme for quad and triangle hanging node resolution

[lenaploetzke]

Closes #2396

Describe your changes here:
This PR adds a general subelement scheme with specializations for quad and triangle hanging node resolution. It also provides two examples and one test.
I implemented only the functions necessary for the visualization. Next steps are all functions necessary for simulations. This means for example neighbors.

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[codecov]

Codecov Report

❌ Patch coverage is 84.27481% with 103 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.35%. Comparing base (ea082ff) to head (ba8cf74).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
.../t8_schemes/t8_subelement/t8_subelement_scheme.hxx	76.40%	80 Missing ⚠️
.../t8_subelement/specializations/t8_scheme_quads.hxx	91.75%	8 Missing ⚠️
...es/t8_subelement/specializations/t8_scheme_tri.hxx	90.69%	8 Missing ⚠️
src/t8_forest/t8_forest_subelement.cxx	93.87%	3 Missing ⚠️
src/t8_schemes/t8_scheme.hxx	85.71%	2 Missing ⚠️
src/t8_forest/t8_forest_adapt.cxx	91.66%	1 Missing ⚠️
src/t8_schemes/t8_subelement/t8_subelement.cxx	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2395      +/-   ##
==========================================
+ Coverage   82.28%   82.35%   +0.07%     
==========================================
  Files         125      131       +6     
  Lines       20703    21352     +649     
==========================================
+ Hits        17035    17584     +549     
- Misses       3668     3768     +100     

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