ADR for model error fourier coefficients

[rdosinski]

Description

This PR contains the ADR document for the additional coeffIndex mars key for the model error fourier coefficients.

Contributor Declaration

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• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.