AP_NavEKF : Add smoothing of GPS and altimeter fusion for velocity and position

This is designed to reduce the level of 5Hz and 10Hz 'pulsing' heard in motors due to GPS and altimeter fusion which casue a 5Hz and 10Hz ripple on the output.
Position and velocity state corrections are applied at a constant rate in the interval from receiving the measurement to the predicted time to receive the next measurement.
The use of time stamps has also been cleaned up along the way