def celltype(row, col, sheet = nil)
      sheet ||= default_sheet
      read_cells(sheet)
      row, col = normalize(row, col)
      @formula[sheet][[row, col]] ? :formula : @cell_type[sheet][[row, col]]
    end
def celltype(row, col, sheet = nil)
      sheet ||= default_sheet
      read_cells(sheet)
      row, col = normalize(row, col)
      @formula[sheet][[row, col]] ? :formula : @cell_type[sheet][[row, col]]
    end
returns the type of a cell:
 * :float
 * :string
 * :date
 * :percentage
 * :formula
 * :time
 * :datetime