Warn when default __init__() modifies C++ attributes

If the C++ object already existed within VTK, and the Python object is being created after the C++ object because it is being returned by a method, then we don't want the creation of the Python object to cause changes to C++ object attributes that were already set by the C++ VTK code.

For example, if we are getting the output dataset from a C++ VTK filter, we don't want any C++ attributes of the dataset to change when the Python portion of this pre-existing dataset object is created.