Add declarative JSON schema for configuring known behaviors

[nrbgt]

Elevator Pitch

Provide a read/write JSON-compatible structure for instantiating and "serializing" the configuration of a forcegraph

Motivation

Having a single, JSON-schema constrained description of a force graph's configuration would simplify a number of use cases.

Additionally, as JSON, it would be theoretically possible to create a mime renderer of a force graph, with this as the metadata. This might be a lower-touch way for other modules to emit a snaphshot of a graph.

Design Ideas

This would allow for something like:

ForceGraph(
    source=src, 
    json={
        "alpha_decay": {},
        "node_labels": {"template": "{{ node.label }}"},
        "node_selection": {"selected": [1,2,3]} 
    }
)

Under the hood these would map to the existing behaviors.

It should then also be possible to extract this back out.

Limitations

It would be challenging to extract de-referenced pointers to the behaviors: this could be somewhat fixed by one of:

• make behaviors an OrderedDict or InstanceDict
• adding a name to each of the behaviors, and exposing them as a "squishy" @property or other getter approach
  • this would then need a @T.validate to ensure each has a unique name