Implements RMST, difference in RMST from the R package:
https://cran.r-project.org/package=survRM2

Includes RMST of one population (point estimate and confidence interval), difference in RMST of 2 populations (point estimate, p-value, and confidence intervals). Addresses CamDavidsonPilon#821

add check on point_in_time argument for difference in RMST

Changes:
1. fixed an issue of returning NaN confidence intervals if the followup time `point_in_time` is greater than the last observed event. Using `.replace(np.inf, 0)`, we correct the issue and return correct confidence intervals for that edge case.
2. cosmetic refactoring to do less recalculation  and rely more on pre-calculated values stored in `fitterA`

…ers get an event_table