AP_GPS: add interface & logging for ublox accuracy estimate

This patch:
- provides an interface to retrieve the speed accuracy (sAcc), horizontal position accuracy (hAcc), and vertical position accuracy (vAcc)
- logs sAcc, hAcc, and vAcc in a new UBX3 dataflash log message

sAcc in particular is going to be very useful for EKF in the immediate term. vAcc may find use in correcting the barometer height. hAcc may find use in, for example, deciding whether we want to set home yet, or to add margin to the geofence.

All of these are in the two messages that we wait on to indicate a new GPS sample, so I didn't need to add any more to that check.