Function pad3d 

fn pad3d(
    a: &[f64],
    from: (usize, usize, usize),
    to: (usize, usize, usize),
) -> Vec<f64>

Zero-pad a column-major 3D array from from to to (extra voxels appended at the high end of each axis), matching numpy.pad(a, [(0, p-d), ...]).