getEdge(vertexIdA, vertexIdB) {
    let edgeId = this.vertexIdsToEdgeId[vertexIdA + '_' + vertexIdB];
    return edgeId === undefined ? null : this.edges[edgeId];
  }
Returns the edge between two given vertices.

@param {Number} vertexIdA A vertex id.
@param {Number} vertexIdB A vertex id.
@returns {(Edge|null)} The edge or, if no edge can be found, null.