update find_level_interface(), refine() for 3D
update find_level_interface(), refine() for 3D

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