Function gamma_process 

pub fn gamma_process(
    rate_hz: f64,
    shape: f64,
    duration_s: f64,
    dt: f64,
    seed: u64,
) -> Vec<f64>

Gamma-renewal spike train (Kuffler et al. 1957). shape=1: Poisson. shape>1: more regular. shape<1: more bursty.