Add Dirichlet process prior to BayesianGaussianMixture (#7386)

doc/modules/mixture.rst

doc/whats_new.rst

examples/mixture/plot_concentration_prior.py

examples/mixture/plot_gmm.py

examples/mixture/plot_gmm_sin.py

sklearn/mixture/base.py

sklearn/mixture/bayesian_mixture.py

sklearn/mixture/dpgmm.py

sklearn/mixture/gaussian_mixture.py

sklearn/mixture/gmm.py

sklearn/mixture/tests/test_bayesian_mixture.py

sklearn/mixture/tests/test_dpgmm.py

sklearn/mixture/tests/test_gaussian_mixture.py

sklearn/mixture/tests/test_gmm.py