add support for threading and for user-specified MPI dimension. Closes #35

[Delcior]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.00000% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 71.46%. Comparing base (ec2b26e) to head (ed0e1b3).

Files	Patch %	Lines
PyMPDATA_MPI/impl/boundary_condition_commons.py	18.75%	13 Missing ⚠️
PyMPDATA_MPI/impl/mpi_boundary_condition.py	80.00%	1 Missing ⚠️
.../local/contract_tests/test_single_vs_multi_node.py	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
- Coverage   71.56%   71.46%   -0.11%     
==========================================
  Files          20       20              
  Lines         422      452      +30     
==========================================
+ Hits          302      323      +21     
- Misses        120      129       +9     

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