Placeholder methods?

[jrfiedler]

Base classes sometimes have placeholder methods that raise a NotImplementedError (for example, CausalEstimator._do), and sometimes they don't (for example, CausalEstimator seems to rely on a method _estimate_effect, but that doesn't have a placeholder method).

Do you generally want the placeholder methods? If so, I can work on finding them and filling them in.

[amit-sharma]

@jrfiedler yes, that's a good idea on adding the placeholders and being consistent. Feel free to add.