fix ProblemDefinition.save_ebc() for EBCs needing ProblemDefinition instance
fix ProblemDefinition.save_ebc() for EBCs needing ProblemDefinition instance

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