Do not differentiate the norm at gimbal lock

At gimbal lock, the norm determining the middle Euler angle is exactly
zero. Its kink carries over through atan2, so the angle has no
derivative. Differentiating the norm anyway produces NaN derivatives.

Compute the norm only away from gimbal lock and use its zero subgradient
otherwise. Likewise, compute the norms in the quaternion manifold test
functor only if they are nonzero.

Change-Id: Ic5c4eaaf1b268b7f002c1489296b212fd45f7727