nodeEdges(v, w) {
    var inEdges = this.inEdges(v, w);
    if (inEdges) {
      return inEdges.concat(this.outEdges(v, w));
    }
  }
Returns all edges to or from node v regardless of direction. Optionally filters those edges
down to just those between nodes v and w regardless of direction.
Complexity: O(|E|).