[variational Bayes] copy all in- and output except data/input_mixture
[GaussianInference] prune according to N_k, not alpha_k

o in practice pruning components with low samples in the latest run
  is better than always considering all runs
o this reverts part of 20c18740f4a892043ac2013b4d8875704357401d
[MixtureDensity] create unsorted samples by default