Change IPTW API

[pzivich]

Lately I have been considering changing the IPTW procedure to more closely adhere to the other estimators. Specifically, IPTW only calculates the weights for the user. It then passes them to the user and assumes they know how to apply them afterwards. I made this decision a long time ago, but am reconsidering.

The API change would be something like the following;

• IPTW gains the marginal_structural_model() function, which allows for specification of the MSM. This would allow users to specify MSM for continuous variables and for interactions

• regression_models() becomes exposure_model() to better align with the other API

• fit() no longer calculates the weights. The weights calculation moves to exposure_model() behind the scenes. Everything still gains the Weight attribute, so users can directly access this and the predicted probabilities

• All assumption checks, like Love plots, would become available after exposure_model() is called.

• Since I would be taking the burden of fitting the MSM, I would need to add different IPTW classes, like SurvivalIPTW for baseline-treatment weighting, LongitudinalIPTW for longitudinal data, and TimeVaryIPTW for time-varying variables.

• Additionally, there would need to be the weight attributed added to allow addition of IPMW or IPCW for estimation procedures.

[pzivich]

Unfortunately, this will be slightly more complicated than I would like. For the all the generalizability/transportability tools for observational data, it takes input of the weights. This aligns with the current API of v0.7.0

I still think estimating marginal structural models for the user is the way forward. To solve this issue, the user could extract the weights from the updated IPTW then supply them to the generalizability tools. I will need to think about this some more...

[pzivich]

Closed via #108