Throttle spools up when in PCL-3/4 while on the ground

[greghgradwell]

With the GPS altitude drift, the estimated altitude can go below the ground, which causes the Tecs.Energy controller to increase throttle.

We need to either implement an arming switch (which was used in the previous autopilot for multirotors), and/or have some sort of "on the ground" logic to disable throttle below a certain speed_cmd threshold.

For now we set throttle to 0 if speed_cmd is < 1.0m/s