Feature: face neighbors and ghosts for handle

[lenaploetzke]

Closes #1945

Describe your changes here:
The mesh handle is now able to get the face neighbors of a mesh element. For this, ghost functionality is needed. ghost elements and mesh elements are now both childs of an abstract element class containing virtual functions for functionality where the element types. Functionality is provided such that users should not have to do type casts.

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[codecov]

Codecov Report

❌ Patch coverage is 98.79518% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.41%. Comparing base (9f807b1) to head (6aedab7).

Files with missing lines	Patch %	Lines
mesh_handle/competences.hxx	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1944      +/-   ##
==========================================
+ Coverage   77.05%   77.41%   +0.36%     
==========================================
  Files         112      112              
  Lines       18959    19034      +75     
==========================================
+ Hits        14608    14736     +128     
+ Misses       4351     4298      -53     

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[sandro-elsweijer]

Will talk to you in person ;)

[sandro-elsweijer]

Same here. Ideally, the user does not need this. Furthermore, it is confusing that an element has a function named get_element. If the element was called element_handle, then this would make sense, since it is a handle to the underlying element

[lenaploetzke]

But this function is in the private area so the user does not see it.