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

(cherry picked from commit cd3b5bf3d98984514dda6945ada63e11568139cd)
Do not replicate fsdat_simul.m (use the one in tests/fs2000 folder).

(cherry picked from commit 534b54074f47a86300f3a22da26f2259e2eae15d)
Added mod files (and associated steadystate files) for k_filter_ahead in the testsuite.

(cherry picked from commit 1fe12d9c3ef9c26c5c7ac6901f06c64ff5e76967)