def labels(_sheet = nil)
      read_labels
      @label.map do |label|
        [label[0], # name
         [label[1][1].to_i, # row
          ::Roo::Utils.letter_to_number(label[1][2]), # column
          label[1][0], # sheet
         ]]
      end
    end
def labels(_sheet = nil)
      read_labels
      @label.map do |label|
        [label[0], # name
         [label[1][1].to_i, # row
          ::Roo::Utils.letter_to_number(label[1][2]), # column
          label[1][0], # sheet
         ]]
      end
    end
Returns an array which all labels. Each element is an array with
 [labelname, [row,col,sheetname]]