Improvement: Implement FetchContent

[ole-alb]

Closes #1723

Describe your changes here:

This PR changes the way how we integrate our submodules. We switch from git submodule to cmake's FetchContent. With this sc and p4est are automatically downloaded when cmake is run the first time.

UPDATE: I also changed that creation of p4est. The release installation of p4est did not link against the release version of sc. I changed it, such that the installation of p4est in release mode links against sc in release mode.

ToDo:

• Update installation guide

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General

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Scripts and Wiki

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License

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[holke]

Please merge with main and see if you can get the tests running.

[Davknapp]

This branch is stalled a bit. However we are further progressing on the FetchContent-Approach with GoogleTest. It would be nice to have the option to use the FetchContent Approach for sc & p4est as well.
As far as I understand the only thing that is required to progress this PR is an update of our CI. What do you need to update the CI?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.95%. Comparing base (6ce35bb) to head (abf8d54).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1722   +/-   ##
=======================================
  Coverage   77.95%   77.95%           
=======================================
  Files         113      113           
  Lines       19089    19089           
=======================================
  Hits        14881    14881           
  Misses       4208     4208           

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