fixed normals for numpy feature of not adding values at repeated indices
fixed normals for numpy feature of not adding values at repeated indices

- also ensured unit norm

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