Vignette packages and files for @kelshmo review

[ewallace]

Responding to @kelshmo's review, ropensci/software-review#470 (comment)

In "Primers and probes calibration vignette" and "Multifactorial multi-plate qPCR analysis example"

• Vignette needs ggplot2 loaded to run. For clarity, I might call library(cowplot) in the first code chunk too.
• /inst will not exist for someone who has installed the package. There are a few references to files in /inst. Instead, you can use system.file() to find this file like you did at the beginning of the vignette: read_lightcycler_1colour_cq( "../inst/extdata/Edward_qPCR_Nrd1_calibration_2019-02-02_Ct.txt" )

@wolski also raised the point about file paths, so that is a priority. See ropensci/software-review#470 (comment)