function GroupedDataFrame(df, columnNames, groups, hashes) {
	    _classCallCheck(this, GroupedDataFrame);

	    this[__groups__] = groups;
	    this[__hashes__] = hashes;
	    this.df = df;
	    this.on = columnNames.length > 0 ? columnNames : df.listColumns();
	  }
Create a GroupedDataFrame. Used in DataFrame.groupBy('columnName').
@param {DataFrame} df The DataFrame to group by.
@param {String} columnName The column used for the group by.
@example
df.groupBy('column1');
//or
groupBy(df, ['column1']);