[vtkusers] Memory Management

[Berk Geveci]

	I think the best way to understand reference counting in vtk
	is to look at the Register and UnRegister methods in 
	vtkObject.cxx (I am looking at  the 3.1 source). These
	are responsible for increasing and decreasing the reference
	count of an object. 

	To see a case where these are used, look at the SetNthInput method
	in vtkProcessObject.cxx. This is where, when an input is added to
	a filter, it is asked to increment it's reference with a
	input->Register(this) call. Some other classes might have
	different implementations and you might need to override this (it
	is virtual) to make it behave like you want.