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 84.81752% with 104 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.45%. Comparing base (85c9c9d) to head (2c806b7).

Files with missing lines	Patch %	Lines
.../t8_schemes/t8_subelement/t8_subelement_scheme.hxx	76.67%	80 Missing ⚠️
.../t8_subelement/specializations/t8_scheme_quads.hxx	91.66%	8 Missing ⚠️
...es/t8_subelement/specializations/t8_scheme_tri.hxx	90.58%	8 Missing ⚠️
src/t8_forest/t8_forest_subelement.cxx	94.80%	4 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   82.36%   82.45%   +0.08%     
==========================================
  Files         125      131       +6     
  Lines       20708    21387     +679     
==========================================
+ Hits        17056    17634     +578     
- Misses       3652     3753     +101     

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[spenke91]

Thanks a lot @lenaploetzke for this great feature! I'm glad to see the subelement concept being molded into an actual implementation 👍
I like the overall code design and think it is great that we barely have to change the interface of the normal schemes. As you will see in one of the comments, I feel like we might be missing one layer of inheritance between "a subelement scheme for triangles" and "a subelement schemes for triangles that resolves haning nodes".

Aside from that, most of my comments are minor suggestion and remarks, as usual :-)

[spenke91]

I would suggest to use more descriptive names for these two files. I know the context is clear from the path, but the path is often not shown, e.g., in vs code. So maybe something like t8_subelement_scheme_tri.hxx or t8_scheme_hanging_nodes_tri.hxx?

[lenaploetzke]

I also like this but i wait for the decision with the abstraction layer before doing this