Update Project 4: Planning.

Vectorize Dubins path computation. This is orders of magnitudes faster than the
previous implementation, so graph caching is now less necessary. As a result,
only graphs for ROS maps are cached; the other test maps are too small to merit
the hassle.

Fix a serious issue with PlanarProblems. Distances were previously measured in
pixels rather than meters. As a result, the connection radius and curvature
parameters didn't have a clear physical interpretation, and would differ based
on the resolution of the map (the conversion factor between meters and pixels).

Add a more efficient edge visualization to RViz, which incrementally sends edges
to be visualized.

Fix minor bugs with zero edges and zero valid samples.

Set a minimum controller speed to avoid issues with reference velocities of
zero. This would occur at the end of a path.