Generates a nrow × ncol matrix with probabilities / frequencies.
If data is given it will be normalized such that sum(data[is.finite(data)])==1.
If no rownames or colnames are given then event names from LETTERS are used.
The returned matrix will have the following attributes:
marginals a list of the row and column marginal distributions
byrow a matrix with conditional probabilities by row
bycol a matrix with conditional probabilities by column
expected a matrix with the expected probabilities under independence
prob a vector of all the probabilities computed (except the expected ones)