Operator + between two CartesianPose is not commutative

[buschbapti]

Because of the nature of the quaternion operation behind the operator+, the operation is not commutative which is slightly weird for an addition. It can lead to weird errors such as:

current_pose = dt * twist + current_pose;

will not lead to desired behavior when

current_pose = current_pose + dt * twist;

does (see #26). This can be a behavior we have to accept but then make super clear that there are differences between them.