AP_NavEKF : Improved GPS glitch detection

This adds new functionality to the detection and compensation of GPS
glitches:

1) A maximum allowable innovation is calculated using the GPS noise
parameter multiplied by the gate, with an additional component allowing
for growth in position uncertainty due to acceleration error since
the last valid measurement
2) If the innovation length exceeds the maximum allowable, no fusion occurs
3) If no innovation has occurred for a long period, then an offset will be
added to the GPS position to make it consistent with the ienertial solution
4) The default velocity innovation gate has been tightened up for copter