inEdges(v, u) {
    var inV = this._in[v];
    if (inV) {
      var edges = Object.values(inV);
      if (!u) {
        return edges;
      }
      return edges.filter(edge => edge.v === u);
    }
  }
Return all edges that point to the node v. Optionally filters those edges down to just those
coming from node u. Behavior is undefined for undirected graphs - use nodeEdges instead.
Complexity: O(|E|).