Improvement: use struct instead of class everywhere

[holke]

Describe your changes here:

Closes #2077

Changes "class" to "struct" throughout the code base (not in mesh handle).
We decided to do this since some of our C code requires structs and classes and structs got mixed up and we encountered some unnecessary compiler errors.
To have a unique solution we switch to struct everywhere.

Please double check that i added the "private/protected/public" modifiers if they were necessary.

I decided against changing the wording of comments. I.e. a comment saying "This class does ..." still reads class, since i feel it is still valid to talk about a struct as a "class" in a comment.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.46%. Comparing base (4665876) to head (19e1357).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2098   +/-   ##
=======================================
  Coverage   77.46%   77.46%           
=======================================
  Files         112      112           
  Lines       19084    19084           
=======================================
  Hits        14784    14784           
  Misses       4300     4300           

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