code: function elasticsearchGlobal (elasticsearchClient) {
      // set client
      if (defined(elasticsearchClient)) {
          // set global client
          ImmutableCoreModel.global().defaultElasticsearchClient =  elasticsearchClient
          // return model when setting value
          return ImmutableCoreModel
      }
      else {
          return ImmutableCoreModel.global().defaultElasticsearchClient
      }
  }
docs: @function elasticsearchGlobal
  
  get/set the elasticsearch global client. returns class instance when
  setting client or client when called with undefined.
  
  @param {object|undefined} elasticsearchClient
  
  @returns {ImmutableCoreModel|boolean|object}