background argument as `surv.sim()` output format

`surv_data` is create in the `bgfscure` package
https://github.com/ICON-in-R/bgfscure

with background rate already appended to each individual.

Instead pass in list of S (and h) for each group and do work inside of main function.
Take structure from `survHE::surv.sim()` output.

This will mean that the background could be supplied by a model fit using `survHE` in the future.


