update ProblemDefinition.create_variables() for no side effects
update ProblemDefinition.create_variables() for no side effects

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