Fix bug in simult_ for purely forward looking models

simult_ simulated one period too much, thinking there was a lag. This resulted in a crash trying to plot irfs.
Fix for theoretical autocovariances for purely forward looking models

In this case, Ex will by empty, leading to non-conformable empty matrices. Solution: compute correction term without using non-existent states.
Add unit tests for stochastic simulations of purely forward looking models

Adds one for purely forward looking models and one for purely forward looking model with additional static variable
Merge pull request #453 from JohannesPfeifer/forward_looking_model_fix

Bugfixes and unit tests for stochastic purely forward-looking models