Introduce proper check for stochastic singularity

In initial_estimation_checks we should introduce a proper check for stochastic singularity by setting options_.use_univariate_filters_if_singularity_is_detected=0 and then providing an informative error message when info(1) == 50. This prevents user errors where the model has a fundamental stochastic singularity, because Dynare by default will resort to the univariate filter without warning.