BUG: Fix proper time dimensionality in categorical Markov chain
ENH: Implement plates to Gaussian Markov chain and relevant unit tests
ENH: For GaussianARD, infer ndim from mu if parent mu is a node
ENH: Solve node plates in constructors

Mixture node needs the parent plates in the constructor, so all
exponential family nodes must compute and return plates in the
constructor.
TST: Add unit tests for Mixture node
BUG: Fix a few plate bugs in categorical Markov chain distribution
DEMO: Add a non-working sketch of a switching LSSM demo