Function py_directed_transfer_function 

pub(crate) fn py_directed_transfer_function(
    py: Python<'_>,
    trains: Vec<PyReadonlyArray1<'_, i32>>,
    bin_size: usize,
    order: usize,
    n_freqs: usize,
) -> (Py<PyArray1<f64>>, usize, usize)