Facets module: adapt to Core changes pt.6 (#699).

Core's `DeepaMehtaUtils`:

`toTopicModels()` takes an `Iterable` instead of a `List`. This allows e.g. to pass a `ResultList` directly (instead of calling `getItems()` on it before).

BREAKING CHANGES

Facets API:

1. The topics returned by the `getFacet()` and `getFacets()` service calls do *not* include their child topics.
When you need the child topics:
    - Java API:
        - on-demand loading (selectively): just call the accessors of the topic's composite value, e.g. `topic.getCompositeValue().getString(...)`. The respective child topics are loaded on-demand.
        - explicit loading:
            - `topic.loadChildTopics()` loads all child topics
            - `topic.loadChildTopics(childTypeUri)` loads child topics selectively
    - REST API: add `fetch_composite=true` to the query parameters. All child topics will be included in the response then.

2. The topic returned by the `getFacettedTopic()` service call does *not* include its child topics.
When you need the child topics: see 1.

3. The `getFacets()` service calls return a `ResultList` (formerly `List`). This allows to call `loadChildTopics()` on it (see 1.) to load all child topics of all items contained in the result list.

See #699.