Support for parameters

[dirk-thomas]

• Nodes contains parameters Implement node parameters and parameter services #214
• Nodes providing ROS services to interact with the parameters Implement node parameters and parameter services #214
• Parameters can be passed through CLI Take initial parameters from parameters yaml files and constructor arguments. #225
• Parameter events are published on updates. Publish parameter events #226
• Nodes can have a parameters_callback set which can arbitrarily accept and reject setting parameters. Add Node API method for setting the parameters_callback. #228
• System tests for rclpy which exercise the parameters API, events, and services Test parameter behavior for rclpy nodes system_tests#293

[nuclearsandwich]

All rclpy parameters related PRs have been merged. Unless there's a reason to keep this tracking issue open, I believe it can be closed.