To allow smtk to manipulate Python operations as C++ objects, we
originally held a reference to the Python operation within the C++
instance to prevent Python from garbage collecting the object. This
resulted in a circular dependency where the Python object would never
be deleted, causing memory overlap issues when multiple Python
operations were used. This MR moves the logic for keeping the Python
representation of an operation into a specialized typecaster for
operations, breaking the cyclic dependency and allowing C++ and Python
to safely free operations when they go out of scope.