vtkQuaternion.txx issue in SLERP interpolation
In vtkQuaternion.txx there is two issues :
1/ The dot product is made between the axes of the rotations represented by the quaternions instead of between the quaternions themself. Hence, the SLERP is made for the rotation's axes and not for the quaternions.
2/ A rotation can be represented by two quaternions (q and -q). To prevent the SLERP to take the long path on the unit sphere we add a check the sign of the dot product. If it is negative, we replace q by -q to take the short path