@Override
    public String toString() {
        return getValueAsStr();
    }
Class to represent a duration. Internally it stores the duration as a {@link Duration} object.
It can be parsed from an ISO-8601 string (e.g. P30D or P1DT6H or PT60S), a stroom {@link ModelStringUtil}
type duration string (e.g. 30d), or a raw milliseconds value.
<p>
Typically suited for durations up to numbers of days.  A day is considered as 24hrs. Does NOT
support ISO 8601 units of Months and Years.
<p>
If {@link StroomDuration} is created from a string then the original string representation
is held and used for later serialisation. This avoids java's desire to serialise P30D as P720H.
<p>
Delegates most methods of {@link Duration} so can be used anywhere a Duration can.
stroom-util/src/main/java/stroom/util/time/StroomDuration.java