ENH: Ensure int type in categorical random sampling
TST: Test initialization of categorical node
ENH: Resolve plates using moments in constant node

Thus, Constant node does not require ndim_observations anymore
because it infers that information from the moment arrays.
TST: Suppress warnings in categorical node unit tests
ENH: Remove ndim_observations dependency from Mixture node class
ENH: Remove ndim_observations from Moments classes
ENH: Remove unused ndims_parents

Mixture node can infer the ndims of parents from parents' dims
attribute.
ENH: Remove ndims attibute from distribution classes
BUG: Fix constructor in Gaussian Markov chains
DEMO: Add no-monitor option to LSSM demo
DEMO: Fix initialization in LSSM-SD demo