Function thd_plus_n_error_db 

pub fn thd_plus_n_error_db(
    wdf: &[f64],
    reference: &[f64],
    fundamental_hz: f64,
    sample_rate: f64,
) -> f64

Compute THD+N error between WDF and reference in dB.

Returns the absolute difference in THD+N between the two signals.