Function find_seed_point 

pub fn find_seed_point(
    mask: &[u8],
    nx: usize,
    ny: usize,
    nz: usize,
) -> (usize, usize, usize)

Find a seed point for region growing: the centre of mass of the mask, or - when that falls outside the mask, as it does for a disconnected or concave one - the nearest voxel that is inside it.