Bug fix: UndirectedGraph does not flag up is_weighted when the edge list contains the weight.

[skojaku]

This PR is to fix a bug of UndirectedGraph object. Namely, when giving an edge list with weights, the flag is_weighted remains "False". The following code can reproduce the error.

import numpy as np
import networkx as nx
from NEMtropy import UndirectedGraph
from scipy import sparse

G = nx.karate_club_graph()
A = nx.adjacency_matrix(G)
labels = np.unique([d[1]['club'] for d in G.nodes(data=True)], return_inverse=True)[1]

src, trg, weights = sparse.find(A)
edgelist = np.vstack([src, trg, weights]).T

graph = UndirectedGraph(edgelist = edgelist)
graph.is_weighted # False

To fix this, I added one line to flag up the weight. See the diff of my edit.