Add constructor of Clustering from class labels

[jg-you]

This would allow the use of clusim as a drop-in replacement for many sklearn.metrics functions.

Not a general constructor, though, since it can't handle overlap or hierarchies.

Related to this PR: it would be fairly easy to add @classmethod decorators to all the .from_X methods to allow a slightly more straightforward initialization as:

clu = Clustering.from_cluster_labels(labels)

instead of the current

clu = Clustering()
clu.from_cluster_labels(labels)

[yy]

Thanks!! @jg-you