cox model updates

[ddsjoberg]

What changes are proposed in this pull request?

• Updated ggsurvfit(), tidy_survfit(), and survfit2() to handle coxph() models. Model must include a strata() component on the RHS of the formula. (#9)

If there is an GitHub issue associated with this pull request, please provide link.

closes #9

library(ggsurvfit)
#> Loading required package: ggplot2
library(survival)

survival::coxph(Surv(time, status) ~ age + strata(sex), data=ggsurvfit::df_lung) |>
  survfit2() |> 
  ggsurvfit() +
  add_confidence_interval() +
  add_censor_mark() +
  add_quantile() +
  add_risktable()

^{Created on 2023-03-06 with reprex v2.0.2}

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