Function directed_transfer_function 

pub fn directed_transfer_function(
    trains: &[&[i32]],
    bin_size: usize,
    order: usize,
    n_freqs: usize,
) -> (Vec<f64>, usize)

Directed transfer function (Kaminski & Blinowska 1991). Returns (d × d × n_freqs) flat Vec.