Enable support for edges to and from subgraphs about psgraph HOT 1 CLOSED

Ok, I have this working now with that syntax. Here is the DOT output:

digraph g {
    compound="true";
    subgraph clusterSource {
        "Source" [label="";style="invis";shape="point";]
        "a" 
    }
    "b" 
    "Source"->"b" [ltail="clusterSource";]
}

With this change, I am injecting in a lot of more DOT instructions.
The subgraph needs to be prefixed with cluster and I was already doing that. I am now injecting a node that matches the name the user gives to the subgraph. This node is small and invisible.

The purpose of this node is so that we can use the name of the subgraph in the edge definition. The user thinks it is pointing to their subgraph but it is really pointing to this invisible node.

To get the line to start or stop at the edge of a subgraph, we need to use the compound="true"; on the base graph with the ltail or 'lhead' attributes on the edge. This does need to match the name of the subgraph.

To do this, I keep a master list of subgraphs created. Whenever an edge is drawn to a subgraph, I use this master list to detect it and inject the needed attributes. This look-up has to happen for every edge connection.

We may see a performance impact from this change as the attributes were preprocessed for an entire list of edges but now I have to do this processing for each edge. Should only be noticeable for large datasets.

from psgraph.