Function signal_model 

fn signal_model(
    t: f64,
    cp: f64,
    cm: f64,
    c0: f64,
    chip: f64,
    chim: f64,
    r0: f64,
    b0: f64,
) -> (f64, f64)

Complex DECOMPOSE signal model at echo time t (seconds). Returns (re, im).