Cleanup element interface

[lukasdreyer]

Describe your changes here:
Several small changes to cleanup the interface of the eclass_scheme, that I put together into one PR, but they are already split in meaningful commits, so I could also do a PR for each one:

Remove t8_element_

• index_array (Follow Todo from file)
• general_function (If the user knows that he has a specific scheme, he can cast his eclass_scheme to that scheme and call additional functionality directly)
• child_eclass (With the current approch, this is always the same as eclass)

Make fam** parameter of is_family const

Remove

• level from t8_element_successor interface. The interesting usecase is to compute the successor on the same level. If the successor of the ancestor or descendant is needed, this can be combined by other elementary functionality (sidenote: we should add a t8_element_ancestor interface function).
• called_new from t8_element_init interface. t8_element_init is not allowed to be called on an element that was created by new.

Introduce function t8_element_deinit. It deinitializes the memory allocated by t8_element_init, but leaves the memory for the element container untouched.

Remove no longer needed debug data from tet elements.

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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Licence

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