[DynareDev] markov_chain statement

Now that we are introducing the multinomial statement that refers to a particular statistical process, I would add 'markov_chain' as an alias for the 'markov_switching' statement, because 'markov_switching' simply defines the Markov Chain process. I would also add an option for a probability matrix and the possibility to calibrate it. Please, tell me what you tink. -- Michel Juillard