Moves an edge so that it targets node_a.

def move_edge_target(self, edge_id, node_a):
        """Moves an edge so that it targets node_a."""
        # Grab the edge
        edge = self.get_edge(edge_id)

        # Remove the edge from the original "target node"
        original_target_node_id = edge['vertices'][1]
        original_target_node = self.get_node(original_target_node_id)
        original_target_node['edges'].remove(edge_id)

        # Add the edge to the new target node
        new_target_node_id = node_a
        new_target_node = self.get_node(new_target_node_id)
        new_target_node['edges'].append(edge_id)

        # Alter the vertices on the edge
        edge['vertices'] = (edge['vertices'][0], node_a)