Model collections

[bsb808]

DRAFT - to be reviewed after #244

Adds launch files to spawn multiple models in one gazebo world for documentation (highlight videos, papers, etc.)

Testing recommendations

1. roslaunch dave_robot_launch uuv_collection.launch paused:=true - see documentation here - https://github.com/Field-Robotics-Lab/dave/wiki/vehicle_examples
2. roslaunch dave_robot_launch glider_collection.launch paused:=true - see documentation here - https://github.com/Field-Robotics-Lab/dave/wiki/Glider-Models

[woensug-choi]

At c4e430e, the propeller orientation and rotation axis are fixed.

Related : #249, #247

PNG Capture:

Rotation Animation GIF capture:

rostopic pub -1 /glider_hybrid_whoi/thrusters/0/input uuv_gazebo_ros_plugins_msgs/FloatStamped "data: 2.0"