ENH: Add architecture for post sim graphics

[zuorenchen]

• Append rocket and balloon trajectories to env.trajectories at each step
• save the trajectories to .json at the end of evaluate_scenario with the save_trajectories function in results/util
• draft render_trajectory_from_file function for future work
• tidy up balloon_world.py

[zuorenchen]

@William-Mou could you help me review this? Feel free to work on the branch directly. Thanks!