Improve example_7: Trajectory generation and plugin

[christophfroehlich]

The IK solver is called with a twist in tool frame and generates a velocity vector in joint space, which is then integrated to positions. That's why the velocity is not zero in the end: one should probably add another field to the message with the last position (is discarded now) and zero velocity.
Maybe the hardware plugin should be improved in a future PR using both position+velocity fields.

Originally posted by @christophfroehlich in #430 (comment)

[devika-dudo]

Hi, is this related to the robot not moving after launching the ``ros2 launch ros2_control_demo_example_7 send_trajectory.launch.py
this command ? I tried building the package and ran the lauch files as told in the docs but the robot isnt moving rviz