This generic method is required for any survival object used in the main
prevalence function.
predict_survival_probability(object, newdata, times)A vector with the same length as times providing the survival probability
estimates.
The survival object itself
Simulated incident individuals, with the same attributes specified in
extract_covars and found in the supplied registry data to prevalence.
The time at which to estimate the survival probability of the individual
in the corresponding row of newdata. Must have as many times as newdata
has rows.