add convert in add and subratc operations

Fix to in Quantity and convert before add or subtract, if these units are different, so now is possible:

        Quantity<Time> day = AbstractQuantity.of(BigDecimal.ONE, SI.DAY);
        Quantity<Time> hours = AbstractQuantity.of(BigDecimal.valueOf(12), SI.HOUR);
        Quantity<Time> result = day.add(hours); //1.5 days


        Quantity<Time> day = AbstractQuantity.of(BigDecimal.ONE, SI.DAY);
        Quantity<Time> hour = day.to(SI.HOUR);//24 hours