Viscous CFL 2D 3D TreeMesh

[DanielDoehring]

This extends #2035 to 2D and 3D TreeMeshes. P4est is supposed to follow in a different PR.

This version implements the CFL criterion only for isotropic diffusive terms, which is fine for the diffusion and NSF equations currently implemented. Thus, this does not work for anisotropic diffusion as in the example

https://trixi-framework.github.io/TrixiDocumentation/stable/tutorials/adding_new_parabolic_terms/#Define-a-new-parabolic-equation-type

See also #2035 (comment)

What we want is an open question.

xref #1147

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 97.64706% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.81%. Comparing base (8b53969) to head (fbe2fc0).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
src/callbacks_step/stepsize_dg2d.jl	92.31%	1 Missing ⚠️
src/callbacks_step/stepsize_dg3d.jl	92.31%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2586   +/-   ##
=======================================
  Coverage   96.80%   96.81%           
=======================================
  Files         530      532    +2     
  Lines       42719    42801   +82     
=======================================
+ Hits        41354    41434   +80     
- Misses       1365     1367    +2     

Flag	Coverage Δ
unittests	96.81% <97.65%> (+<0.01%)	⬆️

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