[cob_footprint_observer] new node providing "safety stop" functionalities based on observed footprint

[mgruhler]

discussion related to #32

This node should get the convex polygon footprint from the footprint observer (see #131) and disable the respective velocity outputs in the twist_mux, once the distance to an obstacle gets below a certain safety distance.

@fmessmer FYI

[fmessmer]

For comparing a footprint representation, i.e. polygon, against the costmap, base_local_planner::CostmapModel seems to provide a function that determines whether the footprint is in collision or not...

Another tutorial on the topic can be found here