[cob_footprint_observer] provide a convex polygon as footprint, not a rectangle

[mgruhler]

Currently, the cob_footprint_observer only outputs a rectangular footprint. This should be adapted to provide a convex polygon. #32

One could think of two possible stages:

1. add dummy links to cover the full size of the robot
2. use the dimension of the meshes somehow

@fmessmer FYI

[fmessmer]

For generating a non-rectangular footprint, i.e. convexHull for a given set of (2D-projected) (tf-Frame) points, we could use cv::convexHull. It also provides cv::minAreaRect and cv::minEnclosingCircle

[fmessmer]

Still investigating possibilities to how to get points, or footprint, properly considering collision meshes