fix time_step_function() to allow presolve if possible
fix time_step_function() to allow presolve if possible

RexFuzzle/sfepy,RexFuzzle/sfepy,lokik/sfepy,BubuLK/sfepy,lokik/sfepy,sfepy/sfepy,RexFuzzle/sfepy,vlukes/sfepy,vlukes/sfepy,rc/sfepy,rc/sfepy,vlukes/sfepy,sfepy/sfepy,rc/sfepy,lokik/sfepy,BubuLK/sfepy,RexFuzzle/sfepy,sfepy/sfepy,BubuLK/sfepy,lokik/sfepy