Standalone scheme face functions

[ole-alb]

Describe your changes here:
Add the face functionality for the standalone scheme.

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)

Tag Label

• The author added the patch/minor/major label in accordance to semantic versioning.

[lukasdreyer]

Had a short look and already stumbled upon 2 things

[Davknapp]

Looks good, but can you comment your code a little bit more? Especially the longer/more complex face functions need more comments.

[Davknapp]

Suggested change

	return 2 * face + ((corner & (1 << face)) >> face);
	T8_ASSERT(element_is_valid(element));
	return 2 * face + ((corner & (1 << face)) >> face);

[Davknapp]

The element itself is actually not needed. Can it be removed from the interface then?

[ole-alb]

The element might be needed in the default scheme and can therefore not be removed from the interface here. This could be addressed in a separate Issue or PR, since a complete removal is out of scope for this PR.