Alt hold expo

KipK was asking for more aggressive control in altitude hold mode. This PR does two changes towards that goal:

1. The stick input is not treated with an exponential when outside the dead band (which is still hardcoded at the inner 10% like previously).
2. Behaves like axis lock mode in terms of transitioning from a position to rate controller when you are requesting much change.

This also has another implicit change. When using this there is no longer the song and dance to enter vario mode as there was previously. It seemed like not many people liked that. If you do not want to have a vario mode then climb rate can be set to zero and it will never enter that mode.  *Do we want to make that the default?*

This has been test flown on Sparky and works quite well.