Add hdr_bin() to convert a continuous variable into discrete hdr areas

[mitchelloharawild]

Used to create the coloured scatterplot.

ggplot(data, aes(x, y, colour = hdr_bin(x, y)) + 
  geom_point()

This may also need to support multivariate high density regions.

[mitchelloharawild]

hdr_bin <- function(x, y, prob, ...){

}

[Sayani07]

Done