Fix PFC on root level elements

[Niklas997]

Closes #2281

Describe your changes here:

When partition for coarsening is applied and the border element is a root level element, a PFC-Message is constructed and the parent element therein is the actual pointer to the root element in the mesh. Upon destruction of the PFC-Message, the parent element (in this case the actual root element in the mesh) is destroyed. In all other cases (i.e. the border element is not a root element) additional storage for the parent is allocated and it is filled with the corresponding information and this newly constructed element will be deallocated within the destructor. Currently, this leads to a crash in case the partition boundary falls on a root element. In order to account for this, there needs to be an additional element-allocation for the parent in the root level case as well which is then filled by an element_copy of the actual root element, such that it can be safely dealloacted upon destruction of the mesasge.

In order to re-create the issue, one can just construct a uniform forest with refinement level 0 from a cmesh and call partiton for caoarsening on it.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.37%. Comparing base (0d30a0b) to head (3d565a1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2280      +/-   ##
==========================================
+ Coverage   82.34%   82.37%   +0.02%     
==========================================
  Files         125      125              
  Lines       20543    20543              
==========================================
+ Hits        16917    16922       +5     
+ Misses       3626     3621       -5     

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[spenke91]

Thanks a lot for catching this! :-) I just have one minor suggestion, hope you don't mind...

[spenke91]

Thanks!