Improved quadrangulated disk mesh cmesh example [6/n]

[jmark]

Describe your changes here:

This is a follow-up to #792.

This PR improves the squared disk cmesh example by rearranging the initial cmesh elements. This should lead
to a better distribution of characteristic element sizes of same refinement level.

Added also a 3D version: cubed sphere

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?
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Scripts and Wiki

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• 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)