Feature: Compute the eclass of a tree or ghost neighbor in the cmesh

[holke]

Closes #1805

Describe your changes here:

Implements t8_cmesh_get_tree_face_neighbor_eclass

Given a local tree or ghost tree and a face this function returns the eclass of the neighbor tree.
If no neighbor is found it returns T8_ECLASS_INVALID

Added the function to test cases that already compute the face neighbor in order to double check the eclass and the new function.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.43%. Comparing base (c9f6591) to head (73e18e4).
⚠️ Report is 62 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1806      +/-   ##
==========================================
- Coverage   74.51%   74.43%   -0.09%     
==========================================
  Files         100      101       +1     
  Lines       19104    19203      +99     
==========================================
+ Hits        14236    14293      +57     
- Misses       4868     4910      +42     

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[Davknapp]

Only a small change required in my opinion! Thanks for the addition.

[Davknapp]

Please assert that face is a positive number, or make it an uint.

[holke]

Done.
Note that t8_cmesh_get_face_neighbor will also performs this check and also check whether face is smaller than the number of faces.
The latter is difficult to check at this place, since we do not have access to the eclass of the tree immediately.
And i figure it is ok to just check against 0 here since t8_cmesh_get_face_neighbor will do the other check.