def summary
      summary = "= #{name}"
      summary << "\n  Number of rows: #{nrows}"
      @vectors.each do |v|
        summary << "\n  Element:[#{v}]\n"
        summary << self[v].summary(1)
      end
      summary
    end
Generate a summary of this DataFrame based on individual vectors in the DataFrame
@return [String] String containing the summary of the DataFrame