use expression materials when copy_materials=True in Problem.create_evaluable()
use expression materials when copy_materials=True in Problem.create_evaluable()

- fixes evaluation of terms with materials not present in Problem.equations
- initialize Problem.conf.materials in Problem.__init__()

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