Add method access Quadtree internals

Add method to access Quadtree internals (Root and for Root its Nodes ). STRTree has such methods (itemsTree, getRoot).

In our case we need it for accessing any element from the Quadtree. With STRTree we can do it like this:

// STRTree st;

Object item = st.itemsTree();

if (!((List<?>) item).isEmpty()) {
  do {
    item = ((List<?>) item).get(0);
  }  while (item instanceof List);
}

// now `item` is a single element from `STRTree`

Start by reading the Quadtree implementation and its Root and Node types, then compare the existing STRTree itemsTree and getRoot methods. Add access to the Quadtree root and its nodes so callers can reach an element, and verify that the resulting traversal supports the use case described in the issue.