Update Project 2: Localization.

Fix bugs in sensor model autograder tests. Specifically, the probability table
was being normalized along the wrong axis.

Add test suite for particle initializer. Add new tests to catch resampling
errors earlier.

Fix small issue in sensor model likelihood plot that affected some maps. Add new
small map to help with sensor model tuning.

Reduce excessive locking in particle filter, motion model, and sensor model.