getEdgeList() {
    let arr = Array(this.edges.length);

    for (var i = 0; i < this.edges.length; i++) {
      arr[i] = [this.edges[i].sourceId, this.edges[i].targetId];
    }

    return arr;
  }
Returns an array containing source, target arrays of this graphs edges.

@returns {Array[]} An array containing source, target arrays of this graphs edges. Example: [ [ 2, 5 ], [ 6, 9 ] ].