NetworkX provides `add_node`, we don't, leads to missing
`graph.node["vop"]`. Does this fix it?

We needed to implement `GraphState.from_json` so that the server can
track a copy of the client's state. That's a useful byproduct of this
work!

`prefix` was used for testing against original implementation

Fixes #8, "Allow control over which `GraphState` operations are depicted by server". Enables use of multiple visualized GraphStates in the same program. Downside -- you have to call `update()` at the end of your program.