Improvement: Renamed forest functions to explicitly say leaf elements

[spenke91]

Closes #1604

Describe your changes here:

Based on the discussion we had regarding issue #1508 , this PR renames some forest-related functions: Their names now expicitly use "leaf_elements" whenever suitable.

Note the related but separate PR renaming some member variables: #1608

All these boxes must be checked by the AUTHOR before requesting review:

• [x] The PR is small enough to be reviewed easily. If not, consider splitting up the changes in multiple PRs.
• [x] The title starts with one of the following prefixes: Documentation:, Bugfix:, Feature:, Improvement: or Other:.
• [x] If the PR is related to an issue, make sure to link it.
• [x] The author made sure that, as a reviewer, he/she would check all boxes below.

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 CMake files.
• [ ] The code is well documented. In particular, 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.

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 or tutorial and a Wiki article.

License

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

[Davknapp]

does it close issue 1508? If so, can please mark it with one of the keywords and delete the automatically opened issue 1607

[spenke91]

does it close issue 1508? If so, can please mark it with one of the keywords and delete the automatically opened issue 1607

Thanks for pointing that out, I already fixed this typo. This PR does not close #1508, but only the subissue #1604. Once both #1604 and #1605 are closed, I will make sure to close #1508