Obstacles to world_model

[garygra]

Example on how to read and update poses of obstacles in prx::world_model_t

[aravindsiv]

Thx for putting this together. For the integration with planning + control, I think it would be useful to have an example of a class that implements:

• a way to update a shared_ptr to the planning_context or world_model_t dynamically so that the replanner is always using the latest information available to it
• this could be either a service/actionlib, or triggered by some condition (like, if the robot is facing the obstacle, then publish its pose)

Let me know if this is too much work for this PR.