Add an agent, connecting it to the previous node.

def add_node(self, node):
        """Add an agent, connecting it to the previous node."""
        other_nodes = [n for n in self.nodes() if n.id != node.id]

        if isinstance(node, Source) and other_nodes:
            raise(Exception("Chain network already has a nodes, "
                            "can't add a source."))

        if other_nodes:
            parent = max(other_nodes, key=attrgetter('creation_time'))
            parent.connect(whom=node)