Fix random state for kmean clustering

When the kmeans algorithm is used to cluster networks, this is not
deterministic by default. The result is that repeated runs of the
`simplify_network` and `cluster_network` rules can and usually do
produce different results that vary somewhat randomly. This makes
results less reproducible when given only a pypsa-eur configuration
file.

The fix is to supply a fixed random state to the k-means algorithm.

scripts/cluster_network.py

@@ -226,6 +226,7 @@ def busmap_for_n_clusters(n, n_clusters, solver_name, focus_weights=None, algori
         algorithm_kwds.setdefault('n_init', 1000)
         algorithm_kwds.setdefault('max_iter', 30000)
         algorithm_kwds.setdefault('tol', 1e-6)
+        algorithm_kwds.setdefault('random_state', 0)
 
     n.determine_network_topology()