Add the ability to add covariates to the model

[amorris28]

Currently, only fits a Surv(age, status) ~ phs model but should allow for adding additional covariates, for example with AD: Surv(age, status) ~ phs + APOE4.

Probably need to add formula interface rather than the simple data, phs, age, status arguments. e.g., so the user can do this:

Surv(age, status) ~ phs + APOE4