This adds PythonTuple and PythonCallable classes to PythonDataObjects.
Additionally, unit tests are provided that exercise this functionality,
including invoking manipulating and checking for validity of tuples,
and invoking and checking for validity of callables using a variety
of different syntaxes.
The goal here is to eventually replace the code in python-wrapper.swig
that directly uses the Python C API to deal with callables and name
resolution with this code that can be more easily tested and debugged.
Note that this patch depends on D14524 (still pending review from Greg) before it can go in.