Compares this Duration to
other, returning zero if the values are equal.
Returns a negative integer if this
Duration is shorter than
other, or a positive integer if it is longer.
Duration is always considered shorter than a positive one.
It is always the case that
duration1.compareTo(duration2) < 0 iff
(someDate + duration1).compareTo(someDate + duration2) < 0.
int compareTo(Duration other) => _duration.compareTo(other._duration);