Feature issue28 get element return const

[holke]

Describe your changes here:

This PR fixes #28.

• t8_forest_get_element now returns a const t8_element_t*
• t8_forest_get_element_in_tree now returns a const t8_element_t*

We do these changes since both getter functions are part of the public interface and users should not be able to modify elements of the forest.

Before merging please double check that this integrates easily into user codes. Especially Trixi @jmark

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

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Github action

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  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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Licence

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

[Davknapp]

Looks good to me, but @jmark should have another look at it with a T8code.jl-lense

[jmark]

@Davknapp @holke Should be fine. The const qualifiers are ignored by the wrappers anyway.