fix NodalLCOperator.__init__() for sympy 0.7.6
fix NodalLCOperator.__init__() for sympy 0.7.6

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