Compensate for nonlinear joystick response.
Move temporary display of left&right out of move()
Re-select when updating so just 1 turtle is appended
Turn 'turtle' into a g element
starting implementation of fading pose trail
switch to using transforms for robot trail
adjust pose trail opacity downward
Add heading indicator to robot 'turtle'
Radius of circle should be *half* the track width
Fiddle with parameters
normalize headings to [-pi, pi]
Add 'wheels' to the current pose turtle