new stat and geom layer checklist

[corybrunson]

This issue will collect stat, geom, or other layers not yet implemented (or found) that should be available for biplots, and for GDA generally, with suggested settings.

If, as proposed in corybrunson/ordr#46, {gggda} is spun off from {ordr}, then this issue should migrate to that package repo. Issue corybrunson/ordr#62 collects layers implemented elsewhere that should be adapted to ggbiplot() layers.

Statistical transformations (stats):

• StatPredBoundary and StatPredRegion (geom = "polygon", mapping = ord_aes(<model>), subset = <variable>) for prediction regions in biplots of categorical variables
• StatBag (geom = "polygon") for point clouds; prototyped by Stephan van der Westhuizen via an answer by Ben at StackOverflow; several methods implemented in {ddalpha}