Support different grouping

[stephenwf]

When you make a query to the search service with a query (however that works) it would be nice to be able to request a particular grouping from the service. Could be driven by different endpoints. For example:

• Group by creator
• Group by manifest (target)
• Group by canvas (target)
• Group by Entity (body)

Ideas of request responses in this case:

{
  "items": [
    {
      "id": "https://omeka.org/api/user/1", 
      "name": "Digirati",
      "annotations": {
        "id": "http://search-service.org/dereferencable-anno-list",
        "type": "AnnotationPage",
        "total": 123
      }
    }
  ]
}

[stephenwf]

They could be attached as annotation collections to IIIF Resources if a particular application needed that functionality.

Collection: Annotations by Users 
    -> Page: List of annotations created by user A
        -> Annotation 1
        -> Annotation 2
    -> Page: List of annotations created by user B
        -> Annotation 1
        -> Annotation 2

[tomcrane]

(this may be the wrong issue for this comment, or it may need a new issue, will move when I've read them all)

We need to balance:

1) the need for a superfast Elasticsearch proxy to Elucidate (where we are well aware of the underlying Elucidate model and capabilities, and are using the search service to accelerate it) 2) the search service as a uniform API abstraction over potentially different sources of data:

• full text from OCR
• static annotations, someone else's annotations
• Elucidate annos 3) for Ghent-like things, for discovery, for collection search; other sources of searchable data altogether - such as descriptive metadata that needs to be shoehorned into an ES schema. Descriptions about the objects as well as content of the object.

use cases indicate we need both - the IIIF Search API is one of the supported query formats of the DLCS