code: async function createElasticsearchIndex () {
      // get client
      var elasticsearchClient = this.elasticsearch()
      // check if index exists
      var exists = await elasticsearchClient.indices.exists({
          index: this.esIndex,
      })
      // skip if index exists
      if (exists) {
          return
      }
      // create index
      await elasticsearchClient.indices.create({
          index: this.esIndex,
      })
  }
docs: @function createElasticsearchIndex
  
  create elasticsearch index if elasticsearch set for model. throws error if
  elasticsearch is true and no client is set.
  
  @returns {Promise}
  
  @throws {Error}