script/extract_surface.py: update for no element groups
script/extract_surface.py: update for no element groups

- use Mesh.from_region()
- allow surface mesh saving also for 2D meshes

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