AP_NavEKF : Fix bug in reset of position and velocity states

If the inertial solution velocity or position needs to be reset to the GPS, the stored state history for the corresponding states should also be reset.
Otherwise the next GPS measurement will be compared to an invalid previous state and will be rejected.
The position state should be reset to a GPS position corrected for velocity and measurement latency. Thsi will make a noticeable difference for high speed flight vehicles, eg 11m at 50m/s
AP_NavEKF : Fix bug in GPS innovation variance growth calculation

The uncertainty in acceleration is currently only scaled using horizontal accelerations, however during vertical plane aerobatics and high g pullups, misalignment in angles can cause significant horizontal acceleration error.
The scaling now uses the 3D acceleration vector length to better work during vertical plane high g maneouvres.
This error was found during flight testing with 8g pullups