add support for issue #17, issue #20, issue #27 and issue #32.

[medxiaorudan]

Hey, I can add more docs to the existing tree by adding these two functions to the RetrievalAugmentation.py and it seems working now, let me know if you need any improvement from here.

  def add_to_existing(self, docs):
       """
       Adds documents to the existing tree and updates the TreeRetriever instance.

       Args:
           docs (str): The input text to add to the existing tree.
       """
       if self.tree is None:
           raise ValueError("No existing tree to add documents to. Please initialize the tree first.")

       # Build tree from the new documents
       new_tree = self.tree_builder.build_from_text(text=docs, use_multithreading=False)

       # Merge the new tree with the existing tree
       merged_tree = self.merge_trees(self.tree, new_tree)

       # Update the tree and TreeRetriever instance
       self.tree = merged_tree
       self.retriever = TreeRetriever(self.tree_retriever_config, self.tree)

   def merge_trees(self, tree1, tree2):
       # Update the all_nodes dictionary of tree1 with the all_nodes dictionary of tree2
       tree1.all_nodes.update(tree2.all_nodes)

       # Update the layer_to_nodes dictionary of tree1 with the layer_to_nodes dictionary of tree2
       for layer, nodes in tree2.layer_to_nodes.items():
           if layer in tree1.layer_to_nodes:
               tree1.layer_to_nodes[layer].extend(nodes)
           else:
               tree1.layer_to_nodes[layer] = nodes

       # Concatenate the root nodes of tree2 with the root nodes of tree1
       if isinstance(tree1.root_nodes, list) and isinstance(tree2.root_nodes, list):
           tree1.root_nodes.extend(tree2.root_nodes)
       elif isinstance(tree2.root_nodes, list):
           tree1.root_nodes = list(tree1.root_nodes.values()) + tree2.root_nodes

       return tree1