insert method
Implementation
insert(RBushBox bbox, T item) {
if (_boxes.containsKey(item)) {
throw StateError(
'Cannot have duplicate $item in the tree, use RBush class for that.');
}
final element = RBushElement(
minX: bbox.minX,
minY: bbox.minY,
maxX: bbox.maxX,
maxY: bbox.maxY,
data: item);
_tree.insert(element);
_boxes[item] = element;
}