ros_control: A generic and simple control framework for ROS

In recent years the Robot Operating System (ROS) has become the 'de facto' standard framework for robotics software development. The `ros_control` framework provides the capability to implement and manage robot controllers with a focus on both _real-time performance_ and _sharing of controllers_ in a robot-agnostic way. The primary motivation for a sepate robot-control framework is the lack of realtime-safe communication layer in ROS. Furthermore, the framework implements solutions for controller-lifecycle and hardware resource management as well as abstractions on hardware interfaces with minimal assumptions on hardware or operating system. The clear, modular design of `ros_control` makes it ideal for both research and industrial use and has indeed seen many such applications to date. The idea of `ros_control` originates from the `pr2_controller_manager` framework specific to the PR2 robot but `ros_control` is fully robot-agnostic. Controllers expose standard ROS interfaces for out-of-the box 3rd party solutions to robotics problems like manipulation path planning (`MoveIt!`) and autonomous navigation (the `ROS navigation stack`). Hence, a robot made up of a mobile base and an arm that support `ros_control` doesn't need any additional code to be written, only a few controller configuration files and it is ready to navigate autonomously and do path planning for the arm. `ros_control` also provides several libraries to support writing custom controllers.