ENH: Force ensureparents decorator to update node attributes
API: Multinomial node takes number of trials as the first argument

Previously, the number of trials was given as an keyword argument
although it was required. Now the syntax is makes more sense and it
resembles the multinomial mathematical syntax.
MAINT: Categorical is now a special case of Multinomial

Categorical node, distribution and moments are subclasses of the
corresponding multinomial classes.
MAINT: Fix typos in Bernoulli unit tests
API: Make the number of trials the first argument in Binomial node

Previously, it was a keyword argument which had to be given. It makes
more sense to have it as a normal argument. In addition, the syntax
resembles the mathematical syntax Bin(n, p).