Add JointDynamicStateInterface.

[olivier-stasse]

This is a request for comment.

This PR is one building block to exploit DynamicJointState (https://github.com/ros-controls/ros2_control/issues/89). The practical issues to be solved are the followings:

• Sending PID values to the power electronics working at a higher frequency (see https://en.wikipedia.org/wiki/Differential_dynamic_programming for a quick introduction, feedback gains)
• New actuators often have additional motors, sensors and it is difficult to use ros_control without breaking the API for one joint.

It is following as much as possible JointStateInterface. My C++ is rusty but I guess it would be possible to filter interface_name using Concept together with Template.