Load a (currently mocked) hierarchical search response in to a tree (acc...

...essible through a root `TreeNode`).

Also, deleted unused class `RefinementList`.

Putting this PR as it would hopefully help someone else to pickup the "build request" ticket (along with some of @danfruehauf's changes in https://github.com/aodn/aodn-portal/tree/new_search_response).

I'm going to go on to the UI - i.e. displaying what's been loaded from XML.