Correctly use MPI communicators

[sblauth]

This PR fixes the behavior of the MPI communicators, which are now correctly inferred from the loaded / created meshes.

[codecov-commenter]

Codecov Report

Merging #181 (ea84dcf) into main (ecbc56c) will decrease coverage by 0.01%.
The diff coverage is 96.00%.

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@@            Coverage Diff             @@
##             main     #181      +/-   ##
==========================================
- Coverage   93.63%   93.63%   -0.01%     
==========================================
  Files          76       76              
  Lines        5825     5836      +11     
==========================================
+ Hits         5454     5464      +10     
- Misses        371      372       +1     

Impacted Files	Coverage Δ
cashocs/_forms/shape_form_handler.py	93.98% <ø> (ø)
cashocs/_forms/shape_regularization.py	95.39% <ø> (ø)
cashocs/_pde_problems/adjoint_problem.py	100.00% <ø> (ø)
cashocs/_pde_problems/control_gradient_problem.py	94.44% <ø> (ø)
cashocs/_pde_problems/hessian_problems.py	100.00% <ø> (ø)
cashocs/_pde_problems/shape_gradient_problem.py	97.33% <ø> (ø)
cashocs/_pde_problems/state_problem.py	98.31% <ø> (ø)
cashocs/geometry/mesh_handler.py	93.39% <ø> (ø)
cashocs/_utils/linalg.py	84.07% <83.33%> (-0.19%)	⬇️
cashocs/_constraints/solvers.py	94.44% <100.00%> (+0.03%)	⬆️
... and 7 more

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