Use Surv objects directly

[aghaynes]

Dear Mark,

I've just had a course on the stata version of stpm2 and thought I'd look into the R version.

Not a problem per se, but something that would increase user friendliness and make rstpm2 more consistent with other packages would be to allow stpm2 to use Surv objects directly (as coxph etc do) rather than having to define it in the formula.

Thanks,

Alan