AP_GPS_GSOF: Supply 5Hz GPS for robust EKF

[Ryanf55]

Feature request

Is your feature request related to a problem? Please describe.

PX-1 provides fused GPS to EKF3 right now, which couples IMU.

Describe the solution you'd like

Add support for a new raw GPS message. We want position, velocity, and 1 sigma accuracy for both pos/vel. Velocity should include horizontal and vertical accuracy.

Describe alternatives you've considered

• Coupling the EKF's

Platform [x] All [ ] AntennaTracker [ ] Copter [ ] Plane [ ] Rover [ ] Submarine

Additional context

N/A