Add SurvivalIPTW and LongitudinalIPTW

[pzivich]

Following #102 with an update toIPTW, the new format is conducive to new classes for estimating IPTW with time-to-event and longitudinal data. Two new classes will be created; SurvivalIPTW for time-to-event data and a singular exposure at baseline. This will replicate Cole and Hernan 2006. The other class with be LongitudinalIPTW which will handle time-varying treatments with longitudinal data (like IterativeCondGFormula does).

This new update structure is preferable to the one mentioned in #75

I plan on adding these classes some time after the v0.8.0 update