fix mat_le_stress(), dw_lin_elastic_iso() for non-constant parameters
fix mat_le_stress(), dw_lin_elastic_iso() for non-constant parameters

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