add dataframe property

[MarcoGorelli]

closes #108

[rgommers]

Thanks Marco. One question: isn't it better to type "returns unknown object" with -> Any rather than -> object?

[MarcoGorelli]

-> object would ensure that no properties of the object are being used - e.g.

from typing import *

class Animal:
    def foo(self) -> object:
        ...
        
class Cat(Animal):
    def foo(self) -> int:
        return 3

reveal_type(Cat().foo())

type checks fine, and would error on Animal().foo().sort_values(), whereas a return type of Any would turn off the type checker and Animal().foo().sort_values() would pass (though that's probably not what we want, because there would be no guarantees about what methods are in the return value from .dataframe)

[rgommers]

Ah fair enough. I haven't seen -> object used before, but the rationale seems solid here. And for users who know what type they want/expect, they can type-narrow themselves.

[rgommers]

This seems straightforward enough and the one minor concern on typing was answered - in it goes. Thanks @MarcoGorelli