AP_GPS_GSOF: Initial EAHRS support

[Ryanf55]

Feature request

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

PX1 is a GNSS-INS, but currently is only treated as a GPS in ArduPilot.

Describe the solution you'd like

• Add an EAHRS driver for the PX-1 that supports ethernet

Describe alternatives you've considered

• n/a

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

Additional context

• Factor out the parser logic from AP_GPS/AP_GPS_GSOF.h into a new AP_GSOF/GSOF.h (NFC)
  • Could split the parser off into its own library
  • Could split the parser off into its own file in AP_GPS and then depend on that in EAHRS
  • Could move the entire device to EAHRS and delete it as a GPS
• Add wiki entry how to configure ethernet transport for GSOF
• Create a new EAHRS driver AP_ExternalAHRS_GSOF for the PX-1 that enables GSOF 49/50 packets
• Add parsers for GSOF 49/50
• Populate EAHRS position/velocity data