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

What is the initial tuning value here? Please provide the value you use in your system, but also a safe value that down weights GPS to a small contribution. We want to avoid getting altitude oscillation issues from bad GPS receivers and so start with a low default.