use Shop::getStress in PeriTriaxController to handle clumps gracefuly
As reported in https://answers.launchpad.net/yade/+question/703276, getStress works for clumps but PeriTriaxController was using a distinct implementation. Fix it.
As reported in https://answers.launchpad.net/yade/+question/703276, getStress works for clumps but PeriTriaxController was using a distinct implementation. Fix it.