update IntegralMeanValueOperator for new base classes
update IntegralMeanValueOperator for new base classes

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