update ConstantFunctionByRegion.get_constants() to use Region.get_cells()
update ConstantFunctionByRegion.get_constants() to use Region.get_cells()

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