code: public List<UnitValue> getFactors(Unit base, RoundingMode mode, Unit...units) {
      List<UnitValue> result = new ArrayList<>();
      for (int i = 0; i < units.length; i++) {
        UnitFactor factor = resolve(units[i], base);
        if (factor != null) {
          BigDecimal n = factor.rational().compute(mode);
          result.add(new UnitValue(n, units[i]));
        }
      }
      return result;
    }
docs: Return an array of factors to convert the array of units to the given base.