def column(column_number, sheet = nil)
      if column_number.is_a?(::String)
        column_number = ::Roo::Utils.letter_to_number(column_number)
      end
      sheet_for(sheet).column(column_number)
    end
def column(column_number, sheet = nil)
      if column_number.is_a?(::String)
        column_number = ::Roo::Utils.letter_to_number(column_number)
      end
      sheet_for(sheet).column(column_number)
    end
returns all values in this column as an array
 column numbers are 1,2,3,... like in the spreadsheet