Robust Variance for Cox (Huber-White Sandwich)

Pivotal Tracker: 51528937

Other authors:
    Qian, Hai <hqian@gopivotal.com>
    Shengwen Yang <syang@gopivotal.com>
    Rahul Iyer <riyer@gopivotal.com>

Changes:
    - Design, and Implementation of Robust Variance for Cox