Add possibility for left-truncation to ppc_km_overlay() and ppc_km_overlay_grouped()#347

jgabry merged 14 commits intostan-dev:masterstan-dev/bayesplot:masterfrom

Sakuski:ppc_km_overlay-left_truncationSakuski/bayesplot:ppc_km_overlay-left_truncationCopy head branch name to clipboard