fix an very important vtkObject memory leak bug
when we call vtkMRMLScene::GetNodesByName, the scene will return a vtkCollection by the follow method
-- vtkCollection* vtkMRMLScene::GetNodesByName(const char* name)
-- {
--   	vtkCollection* nodes = vtkCollection::New();
-- 	...
-- 	return nodes;
-- }
we can see that this function is not responsible for the Delete of the vtkCollection object, so that we must delete the object by ourselves
we introduced this bug in the function stkMRMLHelper::GetSingleMRMLNodeByName and we fix this bug by adding the following line of code
-- listCollection->Delete();