Feature: Add element and user data to mesh handle

[lenaploetzke]

Closes #2006

Describe your changes here:
The mesh handle class is now able to handle element and user data. Template parameters are used to specify the classes (instead of the void* used in the forest for the user data). To specify them as template parameters, i needed to somehow pack the Competences into one template parameter (see parameter_pack.hxx) (because i wanted defaults for the element and user data classes but a parameter pack also always has to be the last template parameter). I think its a feature to have them all in one template parameter for the user and makes it easier to use (although slightly more complicated as a developer). Additionally, i can now provide predefined parameter packs e.g. cache_competences, where all cache competences are included.
The element data is stored in a vector in the mesh (and not in the elements). This is easier for the ghost exchange.
Additionally, the volume of the elements can now be accessed and cached. A tutorial used this as element data which is why i got inspired and also implemented the functionality for the handle. The calculation can be somehow complicated which is why i also provide a cached version.

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

• The PR is small enough to be reviewed easily. If not, consider splitting up the changes in multiple PRs.
• The title starts with one of the following prefixes: Documentation:, Bugfix:, Feature:, Improvement: or Other:.
• If the PR is related to an issue, make sure to link it.
• 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.
• The code coverage of the project (reported in the CI) should not decrease. If coverage is decreased, make sure that this is reasonable and acceptable.
• Valgrind doesn't find any bugs in the new code. This script can be used to check for errors; see also this wiki article.

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).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.16%. Comparing base (d789736) to head (c2a78f5).

Additional details and impacted files

@@                      Coverage Diff                      @@
##           handle-neighbor-and-ghost    #2005      +/-   ##
=============================================================
+ Coverage                      77.12%   77.16%   +0.03%     
=============================================================
  Files                            111      111              
  Lines                          18821    18851      +30     
=============================================================
+ Hits                           14516    14546      +30     
  Misses                          4305     4305              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.