added tinyplot.data.frame method

[zeileis]

Replaces #347

The new data.frame method for tinyplot() draws some inspiration from the corresponding method for base plot(). It mainly facilitates using tinyplot() with data frames and pipes.

tinytheme("clean2")
cars |> tinyplot()

iris |> tinyplot(Sepal.Length ~ Petal.Width | Species)

Currently, this only works for data frames with 1 or 2 variables. In the future we might add a pairs-style display as follows but this would require better handling of axes and their labels.

par(mfrow = c(5, 5))
for (i in names(iris)) for(j in names(iris)) tinyplot(iris[, unique(c(j, i)), drop = FALSE])

So far the PR only contains code and documentation. NEWS and tests can be added when we agree on the code.