Add _semanticPath prop to REST/GraphQL

[etiennedi]

Taken from #1136 which has grown too big. Original input by @bobvanluijt:

---

When Weaviate's semantic features are used to provide results to the end-user, we might be able to improve the UX to why Weaviate came to these results by introducing a __meta "property" that contains additional information per class.

Example API

A query might look like this (also includes the proposed values)

Note: results function just as examples

{
  Get {
    Things {
      Article(
        explore: {
          concepts: ["beatles"]
        }
        limit: 1
      ) {
        title
        _semanticPath {
          path {
            concept
            distanceToQuery
            distanceToResult
            distanceToPrevious
            distanceToNext
          }
        }
      }
    }
  }
}

Todos

• finalize experiments and algorithm to determine meaningful path
• extract into UC
• run once per search item
• add models
• add to GraphQL API
• add to traverser/explorer
• calculate distances
• add appropriate test coverage (how can we test this in a non-flaky fashion?)
• replace go-tsne fork with original (the required PRs have been merged in the mean time)