Remove quaternions

[janden]

These seem have snuck in from the old Matlab code. Specifically the functions qrand, qrand_rots, and q_to_rot should be removed. The only use case is generating random rotations, which can be done using uniform_random_angles and converting from Euler angles using scipy.spatial.transform.Rotation.

[junchaoxia]

@janden Those quaternions function were implemented to reproduce the cov2d example from the Matlab version. I took a look today. We need to remove them from the Cov2D example and unit tests. Of course the unit tests need to be rebuilt. That can generate conflicts with @garrettwrong work. Maybe postpone this?

[janden]

Ok let's leave it for now and come back to it once the row-major stuff is merged.

[junchaoxia]

This issue was fixed in PR 245. Closing now.