Improvement: Element data for mesh handle is now defined via mesh competence

[lenaploetzke]

Describe your changes here:
Closes #2185. Update of #2184 because something got messed up in git.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.49%. Comparing base (8d05ca2) to head (a669865).
⚠️ Report is 16 commits behind head on new-handle-partition-and-balance.

Additional details and impacted files

@@                         Coverage Diff                          @@
##           new-handle-partition-and-balance    #2242      +/-   ##
====================================================================
- Coverage                             78.51%   78.49%   -0.02%     
====================================================================
  Files                                   115      116       +1     
  Lines                                 19196    19207      +11     
====================================================================
+ Hits                                  15071    15076       +5     
- Misses                                 4125     4131       +6     

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[Davknapp]

A first round of comments.

[Davknapp]

I think we don't need to reserve the memory for the vector. std::move should only swap pointers here, so we don't need to allocate new memory. However you should mention in the comment, that element_data is moved, not copied.

[lenaploetzke]

I thought i needed the reserve to avoid a reallocation if we later exchange ghost data (therefore i reserve more space then needed for element_data). What do you think?

[Davknapp]

The move command should be sufficient, if I get it right that this is a std::vector again.
Otherwise I don't realy understand why you first reserve for num_local_elements + num_ghosts and then resize it to num_local_elements. Shouldn't it be enough to just allocate for num_local_elements (if we need an allocation).
If moving is enough, please tell so in the comment.

[lenaploetzke]

Oh yes i dont need the resize (but still reserve for the ghost elements and move, see above)