Add test/support for model_only flag

[dzalkind]

Purpose

Ensure that model_only flag works in WEIS. Add a test to ensure future support. Also testing modeling_overrides in script.

Type of change

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• 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

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Checklist

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• I have run existing tests which pass locally with my changes
• I have added new tests or examples that prove my fix is effective or that my feature works
• I have added necessary documentation

[dzalkind]

@ptrbortolotti and @mayankchetan, I think you use this model_only feature most. Anything else we should do while we're here?

[ptrbortolotti]

this looks good to me! I also see your point of just running a 1 s simulation to make sure that the model actually runs in OpenFAST, so if this model_only flag is painful to maintain, we could also get rid of it