Clean: add comment
New: initialize p_z_xy
Merge branch 'feature_init_p_z_xy' into develop
New: use log probability
Merge branch 'feature_log_probability' into develop
New: learn Estep
Merge branch 'feature_Estep' into develop
New: calculate P(z) in M-step
New: calculate P(x|z) in M-step
New: calculate P(y|z) in M-step
Merge branch 'feature_Mstep' into develop