Function _floyd_warshall_path 

fn _floyd_warshall_path<G, F, K>(
    graph: G,
    edge_cost: F,
    m_dist: &mut Option<Vec<Vec<K>>>,
    m_prev: &mut Option<Vec<Vec<Option<usize>>>>,
) -> Result<(), NegativeCycle>
where
    G: NodeCompactIndexable + IntoEdgeReferences + IntoNodeIdentifiers + GraphProp,
    G::NodeId: Eq + Hash,
    F: FnMut(G::EdgeRef) -> K,
    K: BoundedMeasure + Copy,

Helper that implements the floyd warshall routine, but paths are optional for memory overhead.