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) Per vehicle values for the acceleration error limit have been added
3) If the innovation length exceeds the maximum allowable, no fusion occurs
4) If no fusion has occurred for long enough such that the position uncertainty
exceeds the maximum set by a per vehicle parameter, an offset is applied to the
GPS data to so that it matches the value predicted by the filter
5) The offset is decayed to zero at a rate of 1.0 m/s to allow GPS jumps to
be accomodated gradually
6) The default velocity innovation gate has been tightened up for copter and rover