Fix tidyverse / ggplot2 package imports

[ewallace]

See Using ggplot2 in packages vignette.

[ewallace]

tidyqpcr won't run without features from the tidyverse, including:

• magrittr's pipe
• tibble objects
• many dplyr functions
• ggplot

We need to figure out how to automatically load just as many packages as needed when calling library(tidyqpcr).

[ewallace]

Fixed by ee6d192 in #54, even more fixed by #58. Closing.