Add header for subelements

[chiaraMaHe]

Add a header file for the subelement scheme. Each subelement scheme is based on a element scheme which is used for all non subelement-specific operations.

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.

Licence

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