Function even_odd_ratio_db 

pub fn even_odd_ratio_db(
    signal: &[f64],
    fundamental_hz: f64,
    sample_rate: f64,
    n_harmonics: usize,
) -> f64

Compute even/odd harmonic ratio in dB.

For push-pull amplifiers, even harmonics should be suppressed. A good push-pull stage has ratio < -40 dB.