use a ggplot_add method to simplify ord_aes usage

[corybrunson]

Currently, ggplot_add.LayerRef() enables mapping aesthetics for a reference data set (separate from the inherited data) by storing the aesthetic mapping as a parameter. It is used by two referential stats, StatProjection and StatRule, which process the reference data using StatReferent$setup_params(). This behavior could be spun off to {gggda}. The biplot ggprotos Stat*Projection and Stat*Rule, however, use setup_reference_params(), which handles 'tbl_ord'-specific syntax before passing on to StatReferent$setup_params().

It might be possible for a universal LayerOrd class to additionally obtain the complete set of artificial coordinates from the fortified ordination model (via recover_coord.data.frame()), obviating the need for ord_aes()'s first argument to be the model itself. This class could be automatically built in to all of the (stat|geom)_(rows|cols)_*() shortcuts, resolving the problem of stat_*() rather than geom_*() or even layer() needing to be used, which fails to add the LayerRef class and hopefully triggers this error.