Add simple methods to modify the ROS time

[felixdivo]

Useful features would be:

• skipping ahead (given a time difference)
• skipping back is probably less useful in most scenarios
• setting the time to a concrete value
• waiting for a specific duration/until a certain time point

It would probably be easiest to add them to the env.

[felixdivo]

Useful resources:

• Migration notes from ROS1->ROS2
  • https://github.com/ros-simulation/gazebo_ros_pkgs/wiki/ROS-2-Migration:-ROS-Clocks-and-sim-time
  • ROS Time and use_sim_time ros2/ros2#350
• Design documents
  • https://design.ros2.org/articles/clock_and_time.html
  • Tutorials: Write a ROSTime concept article and tutorial ros2/ros2_documentation#1175
• Implementation
  • https://github.com/ros2/rclpy/blob/f6d652fa2d5d3a8164cc69fb543ae0693e958262/rclpy/rclpy/clock.py#L294

[Timple]

As interface perhaps some setters?

env.time_set(0)
assert node_initalized
env.time_set(18)
assert timeout_behavior

env.time_jump(8.0)  # seconds

env.time_rate(10)  # Runs clock 10x as fast