Integrate Surv() instead of add_duration()?

[andybega]

Integrate the package with the Surv() objects instead of the explicit add_duration function?

This would help match how other survival analysis is done in R.

[andybega]

For backwards compatibility, could leave the existing add_duration(); spdur() interface and add a new splitdur() that uses, maybe, Surv() and also a better formula interface.