Function partial_directed_coherence 

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

Partial directed coherence (Baccala & Sameshima 2001). Returns (d × d × n_freqs) flat Vec.