Clean up nodes on destroy()

[curran]

If property nodes have no incoming or outgoing edges in the data flow graph, they should be removed from the graph when a reactive function that references them (as inputs or outputs) is destroy()ed.

To check:

• inputs with no outgoing edges
• inputs with no incoming edges
• output with no outgoing edges
• output with no incoming edges

When serializing the graph at the last unit test, this is what it looks like now:

{
  "nodes": [
    { "id": "1"  },
    {"id": "2"},
    {"id": "3" },
    ... all the way up to
    {"id": "58" }
  ],
  "links": [
    { "source": "57", "target": 56  },
    { "source": "58", "target": 56 }
  ]
}

[curran]

Necessary for #11

[curran]

Depends on datavis-tech/graph-data-structure#11

[curran]

Required for #7