To make sure that special PyVTKObject attributes (i.e. dict
entries or custom classes) aren't lost when the object goes
out of scope, make a ghost of the object if: a) references
to it still exist within VTK and b) it has custom attributes.
The ghost is brought back to life if the VTK pointer returns
to python.  If the VTK pointer becomes invalid, the ghost is
erased.  Pointer validity is checked via weak pointers.