add a rudimentary docstring to material_opt.py example
add a rudimentary docstring to material_opt.py example

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