def bootstrap(n=nil)
      n ||= nrows
      Daru::DataFrame.new({}, order: @vectors).tap do |df_boot|
        n.times do
          df_boot.add_row(row[rand(n)])
        end
        df_boot.update
      end
    end
Creates a DataFrame with the random data, of n size.
If n not given, uses original number of rows.

@return {Daru::DataFrame}