PID fiddling; visualization

Fiddled around with PID values some more. Set the gain to be much lower (the ratio to convert PID output to motor values), to make the response slower and avoid oscillation while adjusting PID values to shorten the response time.

Also, modularized (somewhat crappily) the visualization and incorporated it into test_stabilize - or rather a new test, test_stabilize_visual. Not yet tested. Hopefully, this should help a lot with the PID tuning process.