Update Project 2: Localization.

Add new autograder tests to catch motion model errors earlier. In particular,
later particle filter tests will fail with a slow unvectorized motion model
implementation.

Update sensor model likelihood plot with a more interpretable default option
that only considers the particle positions (and doesn't encode their angle with
color). The option to color particles by angle is still available with
show_angle=True, and further emphasizes confident particles to make them more
visible.