Fix identification issues around steady state file

The issue popped up in https://forum.dynare.org/t/mode-check-plots-with-flat-lines/16020/13 with the attached files. From what I can see, the issue is parameters being updated in the steady state file. Identification in 4.7 seems unable to handle this. If I am not mistaken (@rattoma should know better), this is a regression as in the past, we resorted to simulation instead of theoretical derivatives which are now unavailable.