human aware velocity and gazing

This pull request introduces two new packages:
* strands_human_aware_velocities: This contains two nodes:
  * human_aware_cmd_vel: This is meant to use the strands_perception_people data to adjust the command vel according to the distance to the closest person.
  * human_aware_planner_velocity: This adjusts the velocity of the DWAPlannerROS according to the distance to the closest found person. This is implemented as an action server.
* strands_gazing:
  * gaze_at_pose: This takes a geometry_msgs/PoseArray and moves the head to gaze at the closest pose. This is implemented as an action server.

For detailed descriptions see readme files.