ENH: Implement random sampling for categorical Markov chain node
TST: Add unit tests for random sampling of categorical Markov chain node
MAINT: Move random sampling methods to Distribution class
MAINT: Add docstrings to Mixture distribution class
ENH: Add random sampling to mixture node
TST: Add unit tests for sampling from mixture node
ENH: Add Hinton diagram for Dirichlet nodes
ENH: Stop VB if converged
DOC: Write discrete HMM example to docs
DOC: Add links to Python and Notebook in HMM example