Added Vector Track and Speed over Ground (VTG) NMEA sentence support to
gdpsdlocatonagent. Velocity location events are used to generate VTG
sentences.

Submitted-by: Leonid Veytser <veytser@ll.mit.edu>
Developed-by: James Kirsch <James.Kirsch@viasat.com>
Merge branch 'feature/vtgnmea' into develop