Adding `n_seeds` and `clusters` members to clusterer

[sbaldu]

This PR adds two memebers to the cluster_properties member of the clusterer:

• clusters, which is an array containing the indices of the found clusters. This is useful to loop over the clusters.
• n_seeds, which is the number of seeds found. The difference with n_clusters is that this doesn't count the set of outliers as a cluster.
• update the equality operator of cluster_properties to also check n_seeds