pairs biplot

[corybrunson]

Because the artificial coordinates of an ordination exist on the same scale, it makes a lot of sense to be able to produce a pairs biplot consisting of pairwise plots of each artificial axis against each other. GGally::ggpairs() might be a good starting point, but a biplot function would ideally return a 'ggplot' object that could accept ordr layers. (A natural name would be ggbipairs().)