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 https://github.com/Cantera/enhancements/issues/22#issuecomment-2025676862.

• 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

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

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