fix compute_nodal_normals() for degenerate elements, check for zero normals
fix compute_nodal_normals() for degenerate elements, check for zero normals

- simplify by using field.get_mapping() instead of custom CMapping
- clean up

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