There was a warning for the Optional initializer having no effect, but
removing the initializer caused the call to add a track to the HashMap
to complain. A constructor looks a little nicer here anyway.
Most users will only care about the total file duration, and shouldn't
be required to determine the file duration from multiple track
durations. To facilitate that, add a total_duration() function that
returns the demuxer's duration not associated to any particular track.
