Update smcure() function

[karissawhiting]

• Update the smcure::smcure() function to save the bootstrap iterations. This will allow us to use these bootstraps to calculate Brier score. (Evacure package has done this already but we may want to keep changes to a minimum: https://github.com/elong0527/evacure/blob/master/R/smcure1.R) - @tengfei-emory
• Look into improving error/warning message on em() function. Right now you get errors or warnings with bootstraps (example coming soon)- @karissawhiting

A certain way of resampling data often causes warning or even an error of the Cox submodel in the cure model. This essentially means the more bootstrap runs we generate, the higher chance of observing a warning or getting an error.

• Once function has been updated, submit issue/PR with suggestion to smcure package authors