arm
- Previously, due to the way MAVSDK's `health_all_ok` was implemented,
vehicle often didn't have a valid global position estimate (although
function returned true), and it wouldn't arm, and the SITL would fail
- Also sometimes as vehicle didn't have manual control, it entered weird
states where it wasn't able to arm as well
- This adds a check to make sure vehicle is able to arm, directly from
the Health struct