Scheme CRTP Update

[sandro-elsweijer]

Describe your changes here:
Draft of a new scheme concept using CRTP. This gets rid of all vtables inside our scheme and implements compile-time polymorphism. Should make all element specific functions faster. This concept also shows, that a templated multilevel conversion scheme is possible.
Drawback of this design is, that every scheme function has to be defined not only in the base scheme, but also in the overall scheme, which holds the individual eclass schemes. This is necessary to hide the CRTP to the user. Otherwise the user would need to use the std::visit function and I want to avoid that:
std::visit is everything wrong with modern C++

todo:

• Rename all ts and tscheme to scheme
• Update the docstrings in t8_element_c_interface
• Rename construct functions
• Decide on how to handle the t8_eclass_scheme class
• Remove if 0 from t8_eclass_scheme
• Decide on what to pass to c interface

All these boxes must be checked by the reviewers before merging the pull request:

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 Makefiles

• The code is well documented

• All function declarations, structs/classes and their members have a proper doxygen documentation

• All new algorithms and data structures are sufficiently optimal in terms of memory and runtime (If this should be merged, but there is still potential for optimization, create a new issue)

Tests

• The code is covered in an existing or new test case using Google Test

Github action

• The code compiles without warning in debugging and release mode, with and without MPI (this should be executed automatically in a github action)

• All tests pass (in various configurations, this should be executed automatically in a github action)

  If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

  • Should this use case be added to the github action?
  • If not, does the specific use case compile and all tests pass (check manually)

Scripts and Wiki

• If a new directory with source-files is added, it must be covered by the script/find_all_source_files.scp to check the indentation of these files.
• If this PR introduces a new feature, it must be covered in an example/tutorial and a Wiki article.

License

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

[Davknapp]

Only some little changes left ;)