methods of convex hull peeling

[corybrunson]

Related to convex hulls and alpha bags, several methods of convex hull peeling have been proposed; one is implemented in the bag branch. Eddy (1982; DOI 10.1007/978-3-642-51461-6_4) cites Green (1980), Chapter 1 in Barnett's Interpreting Multivariate Data, for a thorough review. Eventually, multiple methods should be made available through a common Stat* ggproto with a method argument.

This should probably wait until the transfer to {gggda}.