Honoring `start.time` specifications

[ddsjoberg]

What changes are proposed in this pull request?

• The tidy_survfit() (and subsequently ggsurvfit()) now honor the survfit(start.time) if specified. (#192)

• We now allow for negative follow-up times in tidy_survfit() (and subsequently ggsurvfit()). When negative follow-up times are present users should specify survfit(start.time) and we print a note to this effect when not set. (#192)

If there is an GitHub issue associated with this pull request, please provide link. closes #192

Thank you @bethatkinson for the guidance on this update.

We now respect the start.time specification, and print a note about specifying start.time when there are negative times.

library(ggsurvfit)
#> Loading required package: ggplot2

survfit2(Surv(time, status) ~ sex, df_lung, start.time = 10) |>
  ggsurvfit() +
  add_risktable()

survfit2(Surv(time - 10, status) ~ sex, df_lung) |>
  ggsurvfit()
#> ! Setting start time to -9.83572895277207.
#> ℹ Specify `ggsurvfit::survfit2(start.time)` to override this default.

^{Created on 2024-04-04 with reprex v2.1.0}

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[ddsjoberg]

Thank you for reviewing @bailliem ! Much appreciated!