Add a stratified design example to sim_fix_n

[LittleBeannie]

The following lines of code generate a stratified design simulation of using sim_fixed_n().

This example assumes there are two strata (biomarker-positive and biomarker-negative) with equal prevalence of 0.5 for each. In the control arm, the median survival time is 12 months for biomarker-positive subjects and 8 months for biomarker-negative subjects. For both strata, the hazard ratio is 1 for the first 3 months, after which it decreases to 0.6 for biomarker-positive subjects and 0.7 for biomarker-negative subjects. The dropout rate is contently 0.001 for both strata over time.

sim_fixed_n(
  n_sim = 100,
  sample_size = 500,
  target_event = 350,
  stratum = data.frame(stratum = c("Biomarker-positive", "Biomarker-negative"), 
                       p = c(0.5, 0.5)),
  enroll_rate = data.frame(stratum = c("Biomarker positive", "Biomarker negative"),
                           duration = c(12, 12), rate = c(250 / 12, 250 / 12)),
  fail_rate = data.frame(stratum = c(rep("Biomarker-positive", 2), rep("Biomarker-negative", 2)), 
                         duration = c(3, Inf, 3, Inf), 
                         fail_rate = log(2) / c(12, 12, 8, 8), 
                         hr = c(1, 0.6, 1, 0.7), 
                         dropout_rate = rep(0.001, 4)),
  total_duration = 30,
  block = rep(c("experimental", "control"), 2),
  timing_type = 1:5,
  rho_gamma = data.frame(rho = 0, gamma = 0)
)

It gives me the following error message, which looks like possibly caused by rbinding simulations results?