Interface for any object with dynamic values. This interface can be used for
geographic tracks. It can also be used for tracing the changes to a
measurement over time, or an indexed sequence of words, such as in a news
Get the value in the track at the specified time. If the track is not
timed, this method should return null. If the track is timed, but the
specified time does not match one of the timed coordinates, the value
should be interpolated if possible.