Improvement: Use pragma once in mesh handle and struct instead of class

[lenaploetzke]

Closes #2081

Describe your changes here:
Using #pragma once is now tested in the mesh handle and as decided in the dev meeting, structs are used instead of classes.

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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 (04618cc).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2080   +/-   ##
=======================================
  Coverage   77.46%   77.46%           
=======================================
  Files         112      112           
  Lines       19084    19084           
=======================================
  Hits        14784    14784           
  Misses       4300     4300           

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[spenke91]

Looks all good to me. However, I do have one minor(st) concern you may address or ignore:

I feel like mesh_type and in particular element_type sound somewhat misleading, in that it sounds like they may also just be some integer or something that indicates the type of the element. Do you know what I mean? It's probably not a problem, though 🤔

[spenke91]

👍