remove MassOperator and sfepy/discrete/mass_operator.py
remove MassOperator and sfepy/discrete/mass_operator.py

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