Fully disable 2nd tower mode shapes if DOF is disabled

[dzalkind]

Purpose

In cases where the 2nd tower mode is not critical, it's helpful to just disable it. OpenFAST still checks the mode shapes, and if they are ill-conditioned (usually very large, and still need to sum to 1), an error ensues. This update fills those mode shapes with a placeholder input when the 2nd tower DOF is disabled.

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

Explain the steps needed to test the new code to verify that it does indeed address the issue and produce the expected behavior.

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