Better Manage the ROS dependency

[MaximilienNaveau]

Problem

The code is heavily dependent on the middle-ware ROS. So when/if we switch to another one (ROS2, ...) We will have trouble.

Potential solution

• Create a DGM class without ros.
• Create a class that does the ROS wrapping.
• Remove the ROS entities and create a separate package called dg_ros.