fix Variable.advance() to initialize current step data
fix Variable.advance() to initialize current step data

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