Make T0 and T1 on effect keyword-only to avoid confusion

[kbattocchi]

When using inference, it's somewhat ambiguous whether a call like

est.effect_interval(X, low, high)

uses low and high as the optional treatments or as the lower and upper bounds of the confidence interval (the first interpretation is correct).

This confusion could be avoided by making T0 and T1 into keyword-only arguments, so that the user can call

est.effect_interval(X)
est.effect_interval(X, T0=low, T1=high)
est.effect_interval(X, lower=low, upper=high)

but not

est.effect_interval(X, low, high)

This would be a breaking change, but the safety benefits probably make it worthwhile anyway.

[moprescu]

How does that influence the effect function? I.e. would the user still be able to call effect(x) and effect(x, 0, 1)?

[vasilismsr]

it will have to be effect(x, T0=0, T1=1)

[kbattocchi]

@moprescu, that is a good question.

The simplest approach would be to apply the same rule to effect itself, so that you could call effect(x) and effect(x, T0=0, T1=1) but not effect(x, 0, 1), although this would be a breaking change which otherwise seems unnecessary (since there's no ambiguity here).

Unfortunately, if we want the bootstrap wrapper to remain generic, then it's hard to see how to fix effect_interval without changing effect...

[kbattocchi]

@moprescu The changes in #75 will make these arguments keyword-only for effect_interval while leaving effect untouched by explicitly creating an effect_interval method on BaseCateEstimator.

[vasilismsr]

#75 addresses this