Feature: boundary condition handler MPI communication

[sandro-elsweijer]

Describe your changes here:
The boundary condition handler implemented in #2401 only works for new cmeshes which are created equally on all processes.
This PR introduces a bcast and synchronize function, which can be used for t8_cmesh_bcast or for partitioned cmeshes.
Closes #2405

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[codecov]

Codecov Report

❌ Patch coverage is 87.17949% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.55%. Comparing base (b914808) to head (844babb).

Files with missing lines	Patch %	Lines
...s/internal/t8_cmesh_boundary_condition_handler.hxx	76.92%	6 Missing ⚠️
...s/internal/t8_cmesh_boundary_condition_handler.cxx	95.83%	2 Missing ⚠️
src/t8_cmesh/t8_cmesh_internal/t8_cmesh_commit.cxx	50.00%	2 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                           @@
##           feature-boundary_condition_manager    #2408      +/-   ##
======================================================================
+ Coverage                               82.54%   82.55%   +0.01%     
======================================================================
  Files                                     132      132              
  Lines                                   20998    21073      +75     
======================================================================
+ Hits                                    17332    17397      +65     
- Misses                                   3666     3676      +10     

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