Provide an option to index only if all required fields are found in the triplestore

[nbittich]

it would be useful to be able to only index data with all required fields found in the database, based on role groups.

e.g

     // will not be indexed if given_name is not found (e.g because it's in a different graph not accessible for that role)
      "given_name": {
             required: "true",
             on_path: [
                    "http://www.w3.org/ns/org#heldBy",
                     "http://xmlns.com/foaf/0.1/givenName"
          ],

      }