Add an `R2PptxSlide` object to something else compatible.
# S4 method for R2Pptx,R2PptxSlide
+(e1, e2)An object that a slide can be added to, such as an `R2PptxPresentation`, `R2PptxSlide`, or `R2PptxSlideList`.
`R2PptxSlide` object
If e1 is an object of class R2PptxPresentation then
returns an object of class R2PptxPresentation. Otherwise returns an
object of class R2PptxSlideList