df: base: op: Create output Definition if single value result

[johnandersen777]

Similarly to how we did #594, if we see that there is a type annotation for the return type of a function that op is wrapping, we should create a definition for it. The definition name should be in the form of

'.'.join(func.__module__, func.__qualname__, "result")

The name in kwargs["output"] should be result. The code in op has changed since, #594, check the current code to see how this would be done. Add a testcase to ensure that it's working to tests/test_df.py