def cell(row, col, sheet = nil)
      sheet ||= default_sheet
      read_cells(sheet)
      row, col = normalize(row, col)
      if celltype(row, col, sheet) == :date
        yyyy, mm, dd = @cell[sheet][[row, col]].to_s.split('-')
        return Date.new(yyyy.to_i, mm.to_i, dd.to_i)
      end

      @cell[sheet][[row, col]]
    end
def cell(row, col, sheet = nil)
      sheet ||= default_sheet
      read_cells(sheet)
      row, col = normalize(row, col)
      if celltype(row, col, sheet) == :date
        yyyy, mm, dd = @cell[sheet][[row, col]].to_s.split('-')
        return Date.new(yyyy.to_i, mm.to_i, dd.to_i)
      end

      @cell[sheet][[row, col]]
    end
Returns the content of a spreadsheet-cell.
 (1,1) is the upper left corner.
 (1,1), (1,'A'), ('A',1), ('a',1) all refers to the
 cell at the first line and first row.