def add_data_validation(cells, data_validation)
      dv = DataValidation.new(data_validation)
      dv.sqref = cells
      data_validations << dv
    end
Add data validation to this worksheet.

@param [String] cells The cells the validation will apply to.
@param [hash] data_validation options defining the validation to apply.
@see  examples/data_validation.rb for an example