fix clear_equations() in problemDef.py
fix clear_equations() in problemDef.py

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