Added source and target labels to AbstractEdge.toString().
Changed search providers to return emptySet() to avoid NPEs when centering
on categories with zero vertices.
Added Criteria varargs to a bunch of getVertex() methods in VertexProvider.
Overrode these functions in VertexHopGraphProvider so that vertices that are
derived from CollapsibleCriteria can be fetched from the methods. Cleaned up
VEProviderGraphContainerTest.