Implement filled 2d density contours (#3864)

[sthagen]

• initial implementation idea; still tons of stuff to do

• geom_density2d_filled, doesn't work yet

• fix geom_density2d_filled()

• fix unit test

• update usage examples

• improve documentation

• add news item, allow manual setting of breaks, add nlevel column to contour output

• compute density scaled to samle size, just like in geom_density().

• Make level numeric for filled contours. Fixes Filled contours don't play well with faceting tidyverse/ggplot2#3875.

• Document computed variables.

• properly contour in a two-step process

• cleanup, better docs

• clean-up code, docs

• unit tests

• add a proper stat_density_2d_filled(). Closes stat_density2d doesn't work nicely with geom_polygon tidyverse/ggplot2#3846.

• update news

• fix unit tests

• keep track of the type of contour needed rather than the stat that does the work.

[SimonDedman]

Hi both, sorry to bother you on this pull request thread; I'm trying to change the fill colour from the default viridis in geom_density_2d_filled, and have thrown everything bar the kitchen sink at it, with no joy. Per the note above ("fix geom_density2d_filled()"), is this due to this element not working yet?
If not, (i.e. it is working) any chance someone could pop a quick note on https://ggplot2.tidyverse.org/reference/geom_density_2d.html with an example of the correct format?
FWIW, also on that page, in the "# contour bands and contour lines" section, one doesn't need the "+ geom_density_2d()" line: including "size = 0.25, colour = "black" in the "geom_density_2d_filled" line causes contours to be generated.
Thanks in advance for any intel either way.