Used for the design and analysis of a 2x2 factorial trial for
a time-to-event endpoint. It performs power calculations and significance
testing as well as providing estimates of the relevant hazard ratios and the
corresponding 95% confidence intervals. Important reference papers include
Slud EV. (1994) < https://www.ncbi.nlm.nih.gov/pubmed/8086609>
Lin DY, Gong J, Gallo P, Bunn PH, Couper D. (2016)