bugfix in readings_for_obstacles()
attempt to tweak obstacle radius; helps little
plot llik of observations given ground_gps_pose vs best_particle_pose
refactor to use a simpler representation for trajectories
switch from (pose += noise) to (controls += noise)

Note that last_control_ts was updated incorrectly before (delta_t = 0
except for particles[0]), so the results with (pose += noise) from before
are invalid.