Sightings as vertices in knowledge graph

Like other STIX elements such as Observables, Sightings of Indicators can become vertices in the Stucco knowledge graph.

In PreprocessSTIX, add Sightings as a wrapper and Sighting as an element.
In stucco_ontology.json, add a new section for sightings and update Indicator to have edges to sightings.
In GraphConstructor, add Sighting as a vertex type.

stucco / docs