Function breadth_first_search 

pub fn breadth_first_search<T, S>(
    graph: &S,
    start: usize,
    directed: bool,
    return_predecessors: bool,
) -> SparseResult<(Vec<usize>, Option<Array1<isize>>)>
where
    T: Float + Debug + Copy + 'static,
    S: SparseArray<T>,

Breadth-first search traversal