def << sheet=Workbook::Sheet.new
      sheet = Workbook::Sheet.new(sheet) unless sheet.is_a? Workbook::Sheet
      super(sheet)
      sheet.book=(self)
    end
def << sheet=Workbook::Sheet.new
      sheet = Workbook::Sheet.new(sheet) unless sheet.is_a? Workbook::Sheet
      super(sheet)
      sheet.book=(self)
    end
<< (like in array) a sheet to the workbook (parameter is optional, default is a new sheet)

 @param [Workbook::Sheet] sheet