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 98.71795% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.12%. Comparing base (0573841) to head (d789736).

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

Additional details and impacted files

@@                                Coverage Diff                                @@
##           add-interface-tests-cache-compare-with-forest    #1944      +/-   ##
=================================================================================
+ Coverage                                          76.75%   77.12%   +0.36%     
=================================================================================
  Files                                                109      111       +2     
  Lines                                              18752    18821      +69     
=================================================================================
+ Hits                                               14394    14516     +122     
+ Misses                                              4358     4305      -53     

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