Function derivatives 

fn derivatives(
    s1: f64,
    s2: f64,
    stim1: f64,
    stim2: f64,
    xi1: f64,
    xi2: f64,
    tau_s: f64,
    gamma: f64,
    j_n: f64,
    j_cross: f64,
    i_0: f64,
    sigma: f64,
) -> (f64, f64, f64, f64)