def formulas(sheet = nil)
      sheet ||= default_sheet
      read_cells(sheet)
      return [] unless @formula[sheet]
      @formula[sheet].each.collect do |elem|
        [elem[0][0], elem[0][1], elem[1]]
      end
    end
def formulas(sheet = nil)
      sheet ||= default_sheet
      read_cells(sheet)
      return [] unless @formula[sheet]
      @formula[sheet].each.collect do |elem|
        [elem[0][0], elem[0][1], elem[1]]
      end
    end
returns each formula in the selected sheet as an array of elements
 [row, col, formula]