Create new home for data files used in examples

[speth]

Changes proposed in this pull request

This PR is the implementation of an idea I suggested in Cantera/enhancements#22 (comment).

• Add the newly-created https://github.com/cantera/cantera-example-data repository as a Git submodule
• Depending on the value of the new SCons option example_data, install these data files when installing Cantera
• Refactor the automatic checkout of submodules in SConstruct
• Eliminate the need for the redundant copy of the data files that ends up in build/data

I don't regard this as the final word on which files belong in the data directory of the of this repo and which ones should be moved to cantera-example-data. So far, I've only moved those data files that were only used in examples into the submodule. If we want to move any other files, it will take a bit of work to extricate them from the test suite.

If applicable, fill in the issue number this pull request is fixing

Closes Cantera/enhancements#22

If applicable, provide an example illustrating new features this pull request is introducing

Files from the cantera-example-data repository can be loaded by specifying the path relative to the base Cantera data directory:

gas = ct.Solution('example_data/oxygen-plasma-itikawa.yaml')

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.82%. Comparing base (0c89553) to head (ad64f36).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1689      +/-   ##
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+ Coverage   72.76%   72.82%   +0.05%     
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  Files         378      378              
  Lines       56986    56955      -31     
  Branches    20691    20681      -10     
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+ Hits        41468    41478      +10     
+ Misses      12463    12424      -39     
+ Partials     3055     3053       -2     

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[ischoegl]

Thanks! Minor comment but would also be interested in @bryanwweber’s input.

[bryanwweber]

Looks good to me, aside from the one minor comment!