Consider removing occurrences of aes_string()

[sjspielman]

Currently, many of our plotting functions use aes_string() to handle data mappings. This function has been deprecated in newer ggplot2 versions, so we may want to think about moving away from this strategy and instead using aes() with so-called "curly-curly"'s{{}}.
Although I am opening this issue in this repo, this code change might/is(?) something we should do elsewhere (scpca-nf?), since this repo is more internal. But, I didn't want to forget this so filing here now!

[jashapiro]

We do not currently have any instances of aes_string() in scpca-nf or scpcaTools, so it would just be here for now. But something to keep in mind as we translate any functions from here over to those locations.

Also, given experience with previous soft deprecation in the tidyverse, I do not expect aes_string() to go away for a very long time.