AltitudeHold: allow exponential altitude control

This makes the throttle signal be processed through an
exponential to allow rapid altitude changes but also
sensitivity in the middle range.
AltitudeHold: make vario mode use full state control

In vario mode the desired state is now the current location
with the requested velocity, instead of moving the setpoint.
This behavior is essentially the same as axis lock. For big
stick inputs it enters a rate control and small values it
holds the last position.
Merge remote-tracking branch 'remotes/peabody124/alt_hold_expo' into ltmenabled