Add source_trace.subcircuit_connectivity_map_key

[seveibar]

This would be hugely useful for determining what source traces are connected.

• Add subcircuit_connectivity_map_key to circuit-json
• Add a new render phase that uses circuit-json-to-connectivity-map to compute the keys (note: the keys are like ids except they don't have an associated resource, so that's why we don't call them ids in circuit json)
  • Modify Group.ts to compute this key for all child source traces