fix SimpleTimeSteppingSolver.__call__() for quasistatic problems
fix SimpleTimeSteppingSolver.__call__() for quasistatic problems

- wrong initial step when not loading a restart file caused uninitialized data

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