make generator from .__call__() of time stepping solvers
make generator from .__call__() of time stepping solvers

- yield step, time, state

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