fix find_level_interface() for no vertex-only neighbours in 3D
fix find_level_interface() for no vertex-only neighbours in 3D

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