Get the euclidean distance between a node and its neighbors.

def neighbor_difference(self):
        """Get the euclidean distance between a node and its neighbors."""
        differences = np.zeros(self.num_neurons)
        num_neighbors = np.zeros(self.num_neurons)

        distance, _ = self.distance_function(self.weights, self.weights)
        for x, y in self.neighbors():
            differences[x] += distance[x, y]
            num_neighbors[x] += 1

        return differences / num_neighbors