Allow specifying poses relative to other locations

[sea-bass]

Original comment: "I find it tedious when manually building a world file to have to compute absolute
world coordinates for locations and objects. It would be nice if those could be
specified relative to the parent's center point."

This would require tagging Pose objects with some kind of reference frame a la ROS PoseStamped, and then doing those calculations that way.

It's a great idea, though unclear how much effort it would take.