Keaven's plotgsPower() updates

[jdblischak]

This is an alternative to #126. It is mostly identical. The minor differences are due to:

1. In Plotgspower update #126, Keaven made a few extra edits while fixing the merge conflicts (eg dgt = 4). This PR does not contain those changes
2. In this PR, I was able to keep a final commit from Keaven that he made after merging master. We had removed this commit while preparing Plotgspower update #126. However, I suspect that most of those edits were reverting to changes already in the master branch, because the differences I see locally with git diff HEAD~1 vignettes/SurvivalOverview.Rmd don't appear in this PR

In the end, both this PR and #126 edit the vignette SurvivalOverview.Rmd. We need to pick one and then close the other.

[jdblischak]

Superseded by #126