Ontograph should handle superclasses with no explicit instances

[sa-bpelakh]

If a super-class is present in the class hierarchy, but only instances of its sub-classes are referenced in the graph, there should be a way to visualize the class structure in the graph.

Two possible ways to approach this:

1. Using subgraph per superclass (possibly nesting) to encircle the sub-classes. potentially nesting. This doesn't add extra nodes to the graph, but cannot address removing attribute duplication, removing redundant links and only works with single superclasses.
2. Creating additional nodes (different appearance, the same color as superclass links?) for the superclasses, extracting common data properties from the subclasses into the superclass, and redirecting links from/to the subclasses to the superclass (based on threshold?)