preprocessor:

* In stochastic mode, now transforms the model by removing leads and lags greater or equal to 2 (creating auxiliary variables and equations in the process)
* Information about these variables is in structure M_.aux_vars
* Automatically add the necessary initialization for auxiliary vars after the initval block or load_params_and_steady_state


git-svn-id: https://www.dynare.org/svn/dynare/trunk@3002 ac1d8469-bf42-47a9-8791-bf33cf982152