should Crazyswarm object be a context manager?

[jpreiss]

In #232 we used a Python atexit handler to cleanly shut down the ffmpeg process for video output. This was a good strategy to keep the code changes localized for that commit. However, the most Pythonic way to release resources is with context managers.

I know of one other place in Crazyswarm where we don't technically release resources correctly: we never close the file handle for the Joystick device in genericJoystick.py.

(I am not confident about ROS resources either. What happens if the script process receives SIGINT? I don't know.)

None of this has much impact because we normally only construct one Crazyswarm per process lifetime, with the exception of unit tests. Unit tests don't open the joystick. Normally they don't open ffmpeg either, but for the VideoWriter unit test we needed the hack of spawning a separate process to generate the video and run the atexit handler while the unit test process is still alive.

Making Crazyswarm a context manager won't break existing scripts unless I am missing something.