https://en.wikipedia.org/wiki/K-means_clustering
https://en.wikipedia.org/wiki/Mixture_model
Gaussian Mixture Model (GMM) [coursera.org/learn/robotics-learning]: https://www.coursera.org/learn/robotics-learning/lecture/XG0WD/1-4-1-gaussian-mixture-model-gmm
https://www.quora.com/What-is-the-difference-between-K-means-and-the-mixture-model-of-Gaussian
https://metacademy.org/graphs/concepts/gaussian_mixtures_vs_k_means
http://stats.stackexchange.com/questions/76866/clustering-with-k-means-and-em-how-are-they-related
What are the advantages to using a Gaussian Mixture Model clustering algorithm? – https://www.quora.com/What-are-the-advantages-to-using-a-Gaussian-Mixture-Model-clustering-algorithm
Spectral clustering: https://en.wikipedia.org/wiki/Spectral_clustering