[sandro-elsweijer]

Closes #2567

Describe your changes here:
This PR uses the dispatch method rather than the std:visit method used before. This drastically reduces calls and lookups for the scheme functions.
To make this even more effective, it adds a build option, which links t8code internal functions at link time instead of runtime.
Running the time forest partition benchmark wit the following command results in the following runtimes:

Serial command ./t8_time_forest_partition -b -C -l3 -r3 -x0 -X1 -T1 -D0.1 -o
Parallel command mpirun -np 8 ./t8_time_forest_partition -b -C -l3 -r3 -x0 -X1 -T1 -D0.1 -o -g

Execution mode	Branch	Runtime average over 3 runs	Speedup
Serial	main	12,07 s
	optimized	9,93 s	17,7%
Parallel	main	15,67 s
	optimized	13,43	14,3%

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• The PR is small enough to be reviewed easily. If not, consider splitting up the changes in multiple PRs.
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As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

General

• The reviewer executed the new code features at least once and checked the results manually.
• The code follows the t8code coding guidelines.
• New source/header files are properly added to the CMake files.
• The code is well documented. In particular, all function declarations, structs/classes and their members have a proper doxygen documentation. Make sure to add a file documentation for each file!
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License

• The author added a BSD statement to doc/ (or already has one).

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.54%. Comparing base (980e92a) to head (79ba32b).
⚠️ Report is 292 commits behind head on main.

Files with missing lines	Patch %	Lines
src/t8_schemes/t8_scheme.hxx	96.55%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2566      +/-   ##
==========================================
+ Coverage   83.05%   83.54%   +0.49%     
==========================================
  Files         139      147       +8     
  Lines       21573    21960     +387     
==========================================
+ Hits        17917    18347     +430     
+ Misses       3656     3613      -43     

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[Davknapp]

I think the macro __builtin_unreachable is compiler specific. For our julia- wrapper we have to run it on windows, too.
Can you double-check if this is applicable to windows, too?

[Davknapp]

I added some suggestions to make it more robust and not to trip over something that is only mentioned in the documentation in the future.

All in all: Very nice addition, thak you .