[gnc] Drag force constantly increasing

[tanishm2]

Over time, Fax is increasing in magnitude even though MIDAS is at rest. This is happening in the priori function, in the gnc-data-view branch.

[jjcarsello]

The vel_mag_squared was constantly increasing, even though the midas board is still at rest. We should look through the x_k matrix to see if there are any issues.

[divijgarg]

Key issue is within the priori function. We extrapolate the state on the pad assuming gravity is pulling the rocket down, but this assumption only holds if the rocket is in the air. On the ground, the normal force from the rocket should cancel out the gravitational term, so we need different x_dot functions based on pad vs air.

UPDATE: We do account for this in the priori step in line 140, so I believe this issue is primarily caused within the update function since while our accelerometers measure the gravitational acceleration, we should only be measuring 0 on the pad. Need to add gravity correctly to get this working in update.