allow seeding of init methods in vq.kmeans2

[rgommers]

The vq.kmeans2 function has initialization methods (that can be selected by the minit keyword) that do random number generation without allowing for seeding. See e.g. #9037 (review)

It should be possible for the user to pass in a np.random.RandomState instance or set a seed, rather than having to rely on setting the global state. Can be done in a similar way as the random_state keyword in the stats distributions.