Function par_rows 

fn par_rows<T: Send>(
    a: &mut [T],
    (nx, ny, _): (usize, usize, usize),
    f: impl Fn(usize, usize, usize, &mut [T]) + Sync + Send,
)

f(j, k, l0, row) for each x-row of a volume (l0 its first index; over threads with the parallel feature)