Visualize composition patterns for open models

[epatters]

These are undirected wiring diagrams (UWDs), defined implicitly by variable sharing, as familiar from the @relation macro in Catlab.

In Catlab, UWDs are visualized using Graphviz. That's always an option, and we're already using Graphviz to visualize the generating graph of a model, but I'm hoping we can do better. Specifically, we should investigate Elkjs (#133), which has native support for nested diagrams and will hopefully do better with the outer box of wiring diagrams than Graphviz.

[epatters]

ELK does pretty well on this basic UWD, the composition pattern corresponding to the (undirected) composition of, say, two binary relations.

The source:

algorithm: layered
//hierarchyHandling: INCLUDE_CHILDREN

node outer {
    portConstraints: FIXED_SIDE
    port px {
        ^port.side: EAST
    }
    port pz {
        ^port.side: EAST
    }

    node n1 {
        portConstraints: FIXED_SIDE

        port px {
            ^port.side: EAST
        }
        port py {
            ^port.side: EAST
        }
    }
    node n2 {
        portConstraints: FIXED_SIDE

        port py {
            ^port.side: EAST
        }
        port pz {
            ^port.side: EAST
        }
    }

    node j1 {
        layout [ size: 5,5 ]
    }

    edge n1.px -> outer.px
    edge n1.py -> j1

    edge n2.py -> j1
    edge n2.pz -> outer.pz
}