[CLEANUP] Renam cfgNodeX -> cfgNode.
[CLEANUP] Clean up the Sink class a bit.
[CLEANUP] Add class comment for Sink.
[CLEANUP] Document and simplify SinkProblem.
[CLEANUP] Drop surplus paranthesis and clean up comments in AbstractTacPlace.
[CLEANUP] Improve doc comments of DepedencyGraph.
[CLEANUP] Refactor DependencyAnalysis.
[CLEANUP] Clean up the DepdencencyGraph.
[CLEANUP] Refactor AbstractVulnerabilityAnalysis.