Update graph insert/remove/replace

Created by: thesps

• In remove_node, when the node being removed is an output node, the model outputs need to be reconnected to the input of the removed node.
• In remove_node, we raise an exception that we cannot handle nodes with multiple outputs (nor inputs). However, even for a node with a single output, that output may be consumed my multiple other nodes. So I've added a loop over nodes connected to that output, reconnecting them to the removed node's input whereas previously only one node would have been reconnected
• Rather than replace_node duplicating that logic, I've modified it to use insert_node then remove_node directly.