Using `return_internal_reference` should be invalid for Function

[Kevin Stenerson]

When using `return_internal_reference` (with a CppClassPtrReturnValue), PyBindGen should throw a CodeGenerationError if the return value is for a module Function (or rather, if it is not for a CppMethod).

Otherwise it will produce C++ output which fails to compile with a more cryptic error message.

Example of GCC error message:

pythonBindings.cpp: In function ‘PyObject* _wrap_mymodule_myfunction(PyObject*, PyObject*, PyObject*)’:
pythonBindings.cpp:4015:23: error: ‘self’ was not declared in this scope
     if (((PyObject *) self) && !PySequence_Contains(wards, ((PyObject *) self)))

As a reduced test case, the following should produce some sort of error:

    from pybindgen.module import Module
    from pybindgen import retval

    mod = Module('mymodule')
    mod.add_class('MyClass')
    mod.add_function('myfunction', retval('mymodule::MyClass *', caller_owns_return = False, return_internal_reference = True), [])