getDrawnNeighbours(vertices) {
    let arr = Array();

    for (let i = 0; i < this.neighbours.length; i++) {
      if (vertices[this.neighbours[i]].value.isDrawn) {
        arr.push(this.neighbours[i]);
      }
    }

    return arr;
  }
Returns an array of ids of neighbouring vertices that will be drawn (vertex.value.isDrawn === true).

@param {Vertex[]} vertices An array containing the vertices associated with the current molecule.
@returns {Number[]} An array containing the ids of neighbouring vertices that will be drawn.