query method
Implementation
List<T> query(Rectangle range) {
if (_children.isEmpty) {
return _items
.where((item) => range.containsPoint(item.point))
.map((item) => item.item)
.toList();
}
return _children
.where((child) => child.intersects(range))
.expand((child) => child.query(range))
.toList();
}