[fix] Fixing the namespacing for the sonar plugin

[RaquelTirach]

Namespacing for sonar plugin

While using the gazebo_ros_sonar plugin in a drone URDF model and spawning this type of drone several times in my.world with different robot namespaces, I encountered that type of error, since the same namespacing is used for all sonars on all drones:
[ERROR] [1598280399.141939871, 74.990000000]: Tried to advertise a service that is already advertised in this node [/sonar_front_center_015/range/set_parameters]
I offer here a solution which is highly inspired by the methods GetModelName and GetRobotNamespace from gazebo_ros_utils.h. Those two methods are used in the gazebo_ros_camera plugin and allow several robots from the same model to have their own sensors with their own topics.

[StefanFabian]

Thank you for your PR! This is a good point and your help in creating a solution is greatly appreciated!
However, your proposed change would break code for existing users.
Perhaps it could be enabled using a new option as it was done in #73?

[RaquelTirach]

Thank you for your quick reply!

I updated the pull request to include an option to use the default robotNamespacing. I created a new tag which, if true, enables the default namespacing.

[StefanFabian]

This should be an and &&. Otherwise, the option existing but set to false would not initialize the namespace at all.
Also, I'm pretty sure you also have to extract the value using GetValue()->GetAsBool().

And please follow the code style of the project, e.g., brackets {} on new lines, no spaces between function names and parameters etc.