Fix or document interaction of steady_state_model-block and initval and endval

make_y translates the contents of initval and endval to oo_.endo_simul. The last block's contents are stored in oo_.steady_state. But if there is a steady_state_model-block or a _steadystate-file, we execute in make_y

if options_.steadystate_flag
    [oo_.steady_state,M_.params,~] = ...
        evaluate_steady_state_file(oo_.steady_state,oo_.exo_steady_state,M_, ...
                                   options_,~options_.steadystate.nocheck);
end

This part will overwrite the contents of the block by a computed steady state. This behavior has been in Dynare for at least 10 years, but is clearly wrong. @MichelJuillard @stepan-a Do you know what the purpose of that automatic call to the steady state file is?