fix comsol writer: flatten mat_ids arrays
fix comsol writer: flatten mat_ids arrays

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