EM Algorithm inspired from the article: Dawid and Skene "Maximum likelihood estimation of observer error-rates using the EM algorithm" (1979) http://www.jstor.org/stable/10.2307/2346806

input data= data frame with

• column: worker, with the index of the worker.
• column: item, with the index of the item. note index>=1
• column: label, with the index of the label. note index>=1 (note all the index should start >=1)