swiftnav piksi gps rtk integration in mavlink

Hello Guys!,

It will be cool that we can connect ground piksi gps rtk reference directly to laptop with mission planner and that it communicate over mavlink-3DR Radios to synchro rather than use rtk over other 3dr links...... and also that pixhawk root rtk packet to piksi gps embeded on the multirotor...

The issue does not name files, tests, or entry points. Start by clarifying the intended Piksi GPS RTK, Pixhawk, Mission Planner, MAVLink, and 3DR Radio data flow, then define how synchronization and RTK packet forwarding should work. Done criteria are not specified and need agreement before implementation.

Written by the indexing model from the issue text.