Bugfix in multi us mesh initialization

[anilyil]

Purpose

The initialization of MultiUSMesh makes a few os.system calls to overwrite BCs. In my working docker container, this caused MPI problems where somehow the code was calling MPI init again from the os.system call. To avoid this, I changed the initialization to just load the cgns grid, overwrite BCs, and write the grid back out, which is what cgns_utils does anyways so this is more efficient too.

Expected time until merged

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

Checklist

• I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation

[codecov]

Codecov Report

Merging #81 (a7f36a5) into main (2c4769b) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   73.31%   73.42%   +0.10%     
==========================================
  Files           6        6              
  Lines         757      760       +3     
==========================================
+ Hits          555      558       +3     
  Misses        202      202              

Files	Coverage Δ
idwarp/MultiUnstructuredMesh.py	84.84% <100.00%> (+0.17%)	⬆️

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[ArshSaja]

I ran the multi us mesh with the OWN. It initializes and runs fine.