_getClusterPosition(childNodesObj) {
    const childKeys = Object.keys(childNodesObj);
    let minX = childNodesObj[childKeys[0]].x;
    let maxX = childNodesObj[childKeys[0]].x;
    let minY = childNodesObj[childKeys[0]].y;
    let maxY = childNodesObj[childKeys[0]].y;
    let node;
    for (let i = 1; i < childKeys.length; i++) {
      node = childNodesObj[childKeys[i]];
      minX = node.x < minX ? node.x : minX;
      maxX = node.x > maxX ? node.x : maxX;
      minY = node.y < minY ? node.y : minY;
      maxY = node.y > maxY ? node.y : maxY;
    }

    return { x: 0.5 * (minX + maxX), y: 0.5 * (minY + maxY) };
  }
get the position of the cluster node based on what's inside
@param {object} childNodesObj    | object with node objects, id as keys
@returns {{x: number, y: number}}
@private