getIndex () {
    if (this.parent) {
      const index = this.parent.childs.indexOf(this)
      return index !== -1 ? index : null
    } else {
      return -1
    }
  }
Get the index of this node: the index in the list of childs where this
node is part of
@return {number | null} Returns the index, or null if this is the root node