Trial to implement EM with multiprocessing

Currently, the EM by default runs in sequential mode, i.e., using only a single core of a multicore system. Implementing this using multithreading or multiprocessing would be great.

Here, I started implementing a version using multiprocessing, where you can choose to take a multiprocessing.Pool (for several processes) or a multiprocessing.pool.ThreadPool for multiple threads. Unfortunately it is not working yet, but @tiago.pereira might have an idea how to make it work.