Added GraphViz output for the ReachedSet.

If ReachedSet implements a PartionedReachedSet, a graph is constructed
such that the label for each node shows the contained states.

States can control their output by implementing the
[Graphable] interface.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13767 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c