BUG: Make sure fixed value is a NumPy array before array operations
ENH: Use safe plate resolving in binomial constructor
ENH: Require valid probabilities for fixed Dirichlet
TST: Fix tests to give valid probabilities to Dirichlet nodes
ENH: Check that fixed Dirichlet values are given as a vector
ENH: Implement node class for multinomial random variables
TST: Add unit tests for Multinomial node class