Gathers a decorated data.frame.
I.e. a gather method for class 'decorated'.
Invokes tidyr::gather(), converting gathered
labels to the guide attribute of key, and
converting gathered guides ... if all the same ...
to the guide attribute of value.
Somewhat experimental!
# NOT RUN {library(magrittr)
library(tidyr)
file <- system.file(package = 'yamlet', 'extdata','quinidine.csv')
x <- decorate(file)
x %>% gather('key', 'value', time, interval) %>% decorations
# }