Add 'None' courant field for ShallowWaterScenario tests, change plots in README.md to show MPI calculation in ShallowWaterScenario

[Sfonxu]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.86%. Comparing base (839b4dc) to head (6d4c825).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #178   +/-   ##
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  Coverage   71.86%   71.86%           
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  Files           5        5           
  Lines         263      263           
=======================================
  Hits          189      189           
  Misses         74       74           

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[slayoo]

thanks @Sfonxu
there doesn't seem to be any change influencing MPI subdomain plotting (despite the PR title)?

[Sfonxu]

The code is already there, I just have to get the animation filenames from CI and change the README file.