plot method doesn't return data anymore when type = "hr"

[turgeonmaxime]

When type = "hr", the function plot.singleEventCB use to return the data invisibly, but it doesn't anymore. This can be seen from the vignette: https://cran.r-project.org/web/packages/casebase/vignettes/plotsmoothHazard.html#save-results. This bug was introduced in this commit: 3adaa02

This is an easy fix, but we should add a test so that it doesn't happen anymore.