Make geom_who and friends behave like true geoms

[hafen]

I wrote geom_who() pretty quickly and prior to the ggplot2 extensibility stuff. It would be good to make these geoms behave like true composable geoms.

https://github.com/HBGDki/hbgd/blob/master/R/plot_growthstandard.R#L81-L83

[hafen]

@schloerke you could probably take care of this pretty quickly