Added missing canvas in the case of video-output

[notger]

For the video to be save-able, you need to define the canvas before the FunctionAnimation, but you may only do so for the video output, not for the regular plotting-window output. (Why that is, is beyond me but does not seem worth investigating right now.)

Also, while at it, cleaned up the code a bit and removed unused imports.

Way to verify that the fix works:

> cd centralized/cbs
> python3 ../visualize.py input.yaml output.yaml --video test.mpeg

Solves issue #10 .