def add_row(row_values=[])
      @rows    << Array.new
      row_index = @rows.size-1

      row_values.each_with_index do |row_value, column_index|
        set_cell(row_index, column_index, row_value)
      end
    end
Adds a new row to the data_table.
Call method without any parameters to add an empty row, otherwise, call method with a row object.

Parameters:
* row             [Optional] An array of cell values specifying the data for the new row.
- You can specify a value for a cell (e.g. 'hi') or specify a formatted value using cell objects (e.g. {v:55, f:'Fifty-five'}) as described in the constructor section.
- You can mix simple values and cell objects in the same method call.
- To create an empty cell, use nil or empty string.