Earth mag state covariances are initialized to 0

[CarlOlsson]

@priseborough the issue you are referring to here is not a problem with the new quaternion covariance reset branch but a bug on master.

It was introduced here and happens because when mag_align_complete is set to true here the saved_mag_variance is not updated here so the mag field states are overwritten to zero the first time we activate 3D mag fusion here

One way of solving this is to force 3D fusion for a short time right after takeoff, something a proposed a while back ago here

[priseborough]

Fixed in #556