Fix density calculations for groups with one or two elements

[karawoo]

Fixes #2127.

compute_density() now uses stats::density() to compute the density for two-element groups, resulting in plots that look like this (where group 5 has only two data points):

In the case where there is only a single point in the group (where it doesn't really make sense to calculate density), the group gets dropped with a warning.

## Create some data with a couple very small groups
library("tidyverse")
set.seed(1958)
dat <- tibble(x = c(rnorm(n = 10, mean = 7, sd = 2),
                    rpois(n = 10, lambda = 2),
                    1, 9, 4),
              group = c(rep(c("a", "b"), each = 10), "c", "c", "d"))

## Plot
ggplot(dat, aes(x = x, fill = group)) +
  geom_density(alpha = 0.5)

Warning message:
In compute_density(data$x, data$weight, from = range[1], to = range[2],  :
  Groups with fewer than two data points have been dropped.

There is still a legend element for the dropped group; not sure if that should be kept or not.

[hadley]

Looks good. No need to change the legend.

(In the future, can you please request a review from me? That ensures it gets on my to do list)

[hadley]

Can you please add call. = FALSE here?

[karawoo]

Done!

[hadley]

Thanks!

[lock]

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