Dual treatments

[pzivich]

Currently functionality is not supported for two treatments, like looking at A and B. For binary treatments, this consists of 4 different options.

It may be worth considering the addition of this. Options include: (1) expanding the exposure argument to something like exposure=["A", "B"] or (2) create separate branch for dual treatments specifically but use a similar syntax

I am leaning towards the later. This keeps things consistent with the stochastic treatments and also emphasizes exactly what the user is doing. It would be easier to get lost with option 1 (but I have not decided yet)