ComputingTasks.hh

* Preprocessor: implemented interface to conditional forecast, with three new statements: "conditional_forecast", "conditional_forecast_paths" and "plot_conditional_forecast"
* Reference manual: created a section on forecasting (with "forecast" option), and added the three new statements to that section


git-svn-id: https://www.dynare.org/svn/dynare/trunk@3049 ac1d8469-bf42-47a9-8791-bf33cf982152