Allow single-row values for T0 and T1 when computing the effect (with defaults 0 and 1)

[kbattocchi]

In many cases (single treatment, single response models with binary treatments or linear responses) setting T0 to 0 and T1 to 1 for all rows of the data would be a sensible default that would save users from having to explicitly provide arrays.

[kbattocchi]

@vasilismsr @moprescu
I think this change will be a nice ergonomic improvement. However, should we also then reorder the arguments to effect so that T0 and T1 come last (i.e. effect(X=None, T0=0, T1=1))? Otherwise with the current signature the user would have to specify X as a keyword argument in the common case where it's not None (e.g. myModel.effect(X=X)) because otherwise it would be interpreted as the now optional T0 argument.

[vasilismsr]

Yes. I agree! we should be able to say
model.effect(X)
for binary treatment models