Add omitted 'override' on BMI methods

[PhilMiller]

See NOAA-OWP/ngen#827

Changes

• Add override on all BMI methods, and remove virtual on them, since it would then potentially look like declaration of a new virtual method.

Checklist

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• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
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• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
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