Prius overtake example

[pushkalkatara]

Demo of following example - https://www.youtube.com/watch?v=lBrgfQM1-X0

[okanasik]

Everything looks perfect. After this pull request please send your pull request from VisualStates and VisualStates-examples repos so that I can easily push small changes.
On my computer (maybe because of the bad rendering of intel graphics card), the shadows makes the scene so dark. Also, please make sure that the camera pose of the world files are set such that when a user runs the scene, they can see the whole action without repositionning the camera. To achieve these, can you replace your world file with the following version:

<?xml version="1.0" ?>
<sdf version='1.6'>
  <world name='default'>
    <include>
      <uri>model://sun</uri>
    </include>
    <include>
      <static>1</static>
      <uri>model://cloverleaf_interchange</uri>
      <pose>0 0 0 0 0 0</pose>
    </include>
    <gui>
      <camera name='user_camera'>
        <pose>76 -33 18 0 0.3 0.7</pose>
      </camera>
    </gui>
    <scene>
      <ambient>1.0 1.0 1.0 1.0</ambient>
      <shadows>0</shadows>
    </scene>
  </world>
</sdf>

A small note: when we merge this or crossing example, there will be lots of conflict due to shared prius packages, but I hope we can resolve easily.

[pushkalkatara]

Thanks for the review :D I made the suggested changes in the recent commit.
The changes in the prius plugin and the sdf was to add support for multiple instances of prius in a gazebo world. The previous implementation didn't allow spawning of multiple prius cars, so I added a namespace to each individual car solving the issue. There would be conflicts with the previous example, but yes we'll resolve them with further development.