rMultinom: Generate Multinomial Random Variables with Varying Probabilities

Description

Given a matrix of multinomial probabilities where rows correspond to
observations and columns to categories (and each row sums to 1),
generates a matrix with the same number of rows as has probs and
with m columns. The columns represent multinomial cell numbers,
and within a row the columns are all samples from the same multinomial
distribution. The code is a modification of that in the
impute.polyreg function in the MICE package.