Feature: Add functionality to mesh handle

[lenaploetzke]

Closes #2085

Describe your changes here:
This PR adds multiple functions to the element of a mesh handle, e.g. the diameter and face related functionality as the area of a specific face. We have caches for new functions and new functionality is tested.
Moreover the code is largely restructured to make it shorter and important functions in the element are more visible now.
The testing strategy is sharpened and documented.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.95%. Comparing base (44f449b) to head (c493a05).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2084      +/-   ##
==========================================
+ Coverage   77.56%   77.95%   +0.39%     
==========================================
  Files         113      113              
  Lines       19025    19087      +62     
==========================================
+ Hits        14757    14880     +123     
+ Misses       4268     4207      -61     

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[spenke91]

This is well-written and -documented code, thanks! :-)
I just have a bunch of minor remarks. As usual, feel free to ignore them or argue with me on them -- I think this time I would consider pretty much all of them optional 👍

[spenke91]

Good to go 👍