add option to use stratified response in `surv.glmnet` learners

[bblodfon]

See the vignette here => https://cran.r-project.org/web/packages/glmnet/vignettes/Coxnet.pdf#page=7.43. Seems easy to implement an extra strata variable. But what form is the predict returned object, will have to take a careful look at (eg lp scores, etc)