fix geme_invert3x3() for very small cells
fix geme_invert3x3() for very small cells

- fixes zero basis gradient (singular matrix) problem in reference mappings
  for meshes with very small cells (about 1e-6 edge size in 3D and smaller)

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