AP_NavEKF : Improved GPS glitch detection

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

1) The filters own innovation variance estimate is scaled to calculate
the maximum allowable GPS position innovation
2) If the innovation length exceeds the maximum allowable, no fusion occurs
3) If no fusion has occurred for long enough such that the position uncertainty
exceeds the maximum set by a per vehicle parameter or a maximum time, an offset
is applied to the GPS data to so that it matches the value predicted by the filter
5) The offset is never allowed to be bigger than 100m
6) The offset is decayed to zero at a rate of 1.0 m/s to allow GPS jumps to
be accommodated gradually
7) The default velocity innovation gate has been tightened up for copter and rover