GPS: Propagate RTK fix type to an RTK status
GPS: Reinstate RMC but for date/time only
SCSC: Enable logging at startup rather than arming
GPS: Don't pass high GPS status to AP_Notify

AP_Notify only believes in fixes <= 3D so RTK etc must be mapped down
GPS: Fix glitch detector handling of high fix numbers

It previously tested for != 3D for bad fix, should be < 3D to
account for RTK fixes etc
Arming: Use <3D check for gps unhealthy rather than != 3D
GPS: Add support for offset of GPS from body centre
GPS: Add full DGPS and RTK Float fix types

This is more in line with upstream/master at time of writing (though
they don't support float) and allows better debugging of the dropouts
we're seeing with our new modules
Initial backport of baro drift correction
Baro: Calculate ground level correctly even if we arent on it
Fix units from AHRS to drift comp and to time constant