Fix various bugs related to detrending and prefiltering

• dyn_forecast adds unlogged steady state if loglinear is used
• dynare_estimation_init sets the noconstant based on the unlogged steady state if loglinear is used
• when trends are specified as a function of deep parameters, the values are not correctly updated during estimation due to using the base-workspace parameter values and not the updated local ones
• when using trends with the prefilter option, the mean shift due to the trend is not accounted for
• when using first_obs>1, the higher trend starting point is not taken into account (leads also to problems in recursive forecasting)