fix density definition in units_of_quantities, more num_prefixes
fix density definition in units_of_quantities, more num_prefixes

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