Add 3d subcell positivity limiting for nonlinear variables

[bennibolm]

After the initial 3d subcell limiting PR #2582, this PR adds positivity limiting for nonlinear variables.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.85%. Comparing base (867d2fa) to head (dbe728f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/solvers/dgsem_p4est/subcell_limiters_3d.jl	94.29%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2647      +/-   ##
==========================================
+ Coverage   95.12%   96.85%   +1.73%     
==========================================
  Files         546      546              
  Lines       43204    43246      +42     
==========================================
+ Hits        41096    41885     +789     
+ Misses       2108     1361     -747     

Flag	Coverage Δ
unittests	96.85% <95.12%> (+1.73%)	⬆️

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[bennibolm]

I think these are so little additions that we can move on and merge this PR without @amrueda's final look. Are you okay with that @DanielDoehring ?

[DanielDoehring]

I think these are so little additions that we can move on and merge this PR without @amrueda's final look. Are you okay with that @DanielDoehring ?

Sure 👍