Use ein sum in flux term.
Use ein sum in flux term.

This seems to speedup flux calculation 10 times on 2D tensor prod mesh,
untested elsewhere.

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