template<typename GraphTraits>
class cluster< GraphTraits >
Abstract base class for splitting dnodes into hierarchical clusters
in the generated .dot file.
See "Subgraphs and Clusters" within
https://www.graphviz.org/doc/info/lang.html
and e.g.
https://graphviz.gitlab.io/_pages/Gallery/directed/cluster.html
If a root_cluster is passed to dump_dot*, then all nodes will be
added to it at the start of dumping, via calls to add_node.
The root cluster can organize the nodes into a hierarchy of
child clusters.
After all nodes are added to the root cluster, dump_dot will then
be called on it (and not on the nodes themselves).