Add cmesh example cubed spherical shell [3/n].

[jmark]

Describe your changes here:

This is a follow-PR to #787.

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)

[jmark]

@sandro-elsweijer I assigned you this PR since it is nearly identical to #787 and should not add much workload to review it.

[sandro-elsweijer]

I would find it better to adapt the t8_cmesh_new_quadrangulated_spherical_surface routine from your previous PR. We could add a 2D/3D flag and then just add another loop for the radial layers.
The approach to make an independent forest seems to be overkill and would involve more code maintenance in the future