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)