Fix memory leak for Python reference parameters

The docs for PyVTKReference say that PyVTKReference_SetValue(self, o)
steals the reference for 'o', but that is not how it was implemented.
This commit fixes the implementation so that it matches the docs.

Wrapping/PythonCore/PyVTKReference.cxx

@@ -172,6 +172,7 @@ int PyVTKReference_SetValue(PyObject* self, PyObject* val)
     PyObject** op = &((PyVTKReference*)self)->value;
 
     PyObject* result = PyVTKReference_CompatibleObject(self, val);
+    Py_DECREF(val);
     if (result)
     {
       Py_DECREF(*op);