Develop gs_update_ahr for boundary update at time of analyses

[LittleBeannie]

For an original design created by gs_design_ahr or gs_power_ahr, the efficacy or futility need updates when the numbers of observed events are different from the plan ones. The object of gs_update_ahr is to update the original design with updated boundaries, as well as the crossing probability of the new boundaries under both H0 and H1. We will also have a corresponding as_gt function for gt display of the updated design.