Replace initval blocks by steady_state_model blocks in fs2000 unit tests

tests/AIM/fs2000_b1L1L.mod

tests/AIM/fs2000_b1L1L_AIM.mod

tests/AIM/fs2000_b1L1L_AIM_steadystate.m

tests/AIM/fs2000_b1L1L_steadystate.m

tests/Makefile.am

tests/analytic_derivatives/fs2000_analytic_derivation.mod

tests/conditional_forecasts/fs2000_cal.mod

tests/conditional_forecasts/fs2000_est.mod

tests/dates/fs2000.mod

tests/estimation/fs2000.mod

tests/estimation/fs2000_MCMC_jumping_covariance.mod

...s/estimation/fs2000_MCMC_jumping_covariance_steadystate.m

tests/estimation/fs2000_calibrated_covariance.mod

tests/estimation/fs2000_initialize_from_calib.mod

tests/estimation/fs2000_initialize_from_calib_steadystate.m

tests/estimation/fs2000_mf.mod

tests/filter_step_ahead/fs2000_filter_step_ahead_ML.mod

...lter_step_ahead/fs2000_filter_step_ahead_ML_steadystate.m

...s/filter_step_ahead/fs2000_filter_step_ahead_bayesian.mod

...tep_ahead/fs2000_filter_step_ahead_bayesian_steadystate.m