Group artifacts by groupId

[vgheo]

In large projects, it may be useful to present artifacts with the same groupId into a graphviz cluster.

As configuration API I think this could be implemented by adding a property 'clusterId' on each node, that would be used as cluster name in the dot output.

eg - config

dependencyNode = { node, dependency -> node.add( ClusterId.of(dependency.getModuleGroup()) }

output

subgraph cluster_comexample {
"comexamplea"
"comexampleb"
}