Adding vertical (Z) velocity to inav estimator

This is needed for high-accuracy GNSS sensors (like Ashtech MB100 we are using). It allows copter to maintain better altitude stability.

Basically copying horizontal code.

Here is the log http://dash.oznet.ch/view/wxpvxAKYstGZjXvkKTSkye

We are using value 1.0 now even without RTK differentials, and I'd guess it would be quite safe to start from 0.5. My understanding is, since most GNSS receivers provide vertical measurements twice as inaccurate as their horizontal measurements, it ought to be 2 times less than INAV_W_XY_GPS_V.