Fix bug in Bayesian estimation where the filter_step_ahead command produced no output

- Improves documentation of resulting matrices
- Adds unit test for filter_step_ahead option
Added the possibilty to set the initial condition of the (stochastic) extended path simulations with the histval block.
Do not replicate fsdat_simul.m (use the one in tests/fs2000 folder).
Added mod files (and associated steadystate files) for k_filter_ahead in the testsuite.