Resize ground to the size of the city

[Javier Iván Choclin]

It is necessary so it is easier to move in the Gazebo world

[Agustin Alba Chicar]

Tried with the following:

• Getting a valid reference to the world pointer via gazebo::physics::get_world() but I couldn't even compile it.
• Changing the sdf definition that is loaded and changing the model of the ground --> it fails
• Spawning the ground models --> it fails, I can't even set the name
• Spawning boxes --> it fails, not code execution, but the result, no boxes were visible.

@JChoclin I'll put this in hold state until we move to the WorldPlugin (#47)

[Agustin Alba Chicar]

This can be done directly from the generated world file as the generator has all the information.