Respect the use_sim_time parameter for rclpy nodes

[dhood]

There is support for Clocks that use ROS Time as described in this design document. It was added in this PR.

Clocks can be set to have ROS time "active" (will use the value seen on the /clock topic as its time) by calling time_source.ros_time_is_active = True.

A node has a time source associated with it (added in #210). At the time of that PR, parameters were not available in rclpy, so the node's time source does not respond to the use_sim_time parameter:

rclpy/rclpy/rclpy/node.py

Line 97 in 62012d3

# TODO(dhood): use sim time if parameter has been set on the node.

As of #214, parameters are available in rclpy.

Tasks to get rclpy nodes to respond to the use_sim_time parameter:

• Add a parameter callback that detects if use_sim_time has been set on the node.
• Add tests that the node's clock output matches what's expected based on the combination of use_sim_time parameter and the presence of a /clock publisher. These are the tests in rclcpp.

[shivangg]

@dhood I'd like to work on this issue. Can you assign it to me?

[sloretz]

@shivangg Thanks!

I can't assign the issue to you (isaacs/github#100), but your contribution is still very welcome. When you make a pull request add the text connects to ros2/rclpy#240 to the PR description. That will link the PR and issue on the waffle board.

[mkhansenbot]

@shivangg - I'm in need of this functionality now for testing ROS2 Navigation, did you get this to work?

[jacobperron]

Resolved by #297