code: function() {
          var columnData = this._getColumnData();
          var columnWidths = columnData.widths;
          var columns = columnData.columns;
          var htmlList = [];
  
          _.each(columnWidths, function(width, index) {
              htmlList.push(this.templateCol({
                  attrColumnName: ATTR_COLUMN_NAME,
                  columnName: columns[index].name,
                  width: width + CELL_BORDER_WIDTH
              }));
          }, this);
  
          return htmlList.join('');
      }
docs: col group 마크업을 생성한다.
  @returns {string} <colgroup>에 들어갈 html 마크업 스트링
  @private