Adds yaw offset property

[Rauldg]

since the init pose property ignores the orientation part of the pose

[chhtz]

I would kind-of prefer fixing the start_pose functionality. E.g., just always output start_pose * odo_pose (where odo_pose is what is currently calculated when start_pose is not set). But then renaming start_pose to pose_offset would probably make more sense. And it would be something which would also be achievable with the transformer (probably not as convenient as with a property).

I'll ask some other SLAM users for their opinion.

[planthaber]

#7