function ll = joint_log_lik(doc_counts,topic_counts, alpha, gamma)
%Gets the joint log likelihood of the model
%
% @param doc_counts         : n_docs x n_topics vector of counts per document
%                           of unique topics
% @param topic_counts       : n_topics x alphabet_size vector of counts per topic of unique
%                           words
% @param alpha              : prior dirichlet parameter on document specific
%                           distributions over topics
% @param gamma              : prior dirichlet parameter on topic specific
%                           distribuitons over words.
%
%@return ll                 : joint log likelihood of model
