Create more than one track (or segment) per GPX file

[ejain]

I don't always start logging where I last stopped logging. If the data exported to GPX had one track (or segment) for each period of recording, that would avoid some ugliness when trying to look at the data in apps like Google Earth. The docs do mention that GPSLogger "logs GPX points as tracks and track segments"--I've just never seen the segments happen...