ensure nationality instances are classified as Nationalities

[azaroth42]

Slight refactor to the vocab processing order to allow instances to use the pseudo-classes defined in ext_classes.

Now nationalities come out as:

  "@context": "https://linked.art/ns/v1/linked-art.json", 
  "id": "http://vocab.getty.edu/aat/300107956", 
  "type": "Type", 
  "_label": "American", 
  "classified_as": [
    {
      "id": "http://vocab.getty.edu/aat/300379842", 
      "type": "Type", 
      "_label": "Nationality"
    }
  ]
}

As described here: https://linked.art/model/actor/#nationality