Return a SlideCatalog from a CrunchDeck. All slide catalog methods should be
available for CrunchDecks, but this function is used internally to model the
API.
slides(x)
slides(x) <- value
# S4 method for class 'CrunchDeck'
slides(x)
# S4 method for class 'CrunchDeck'
slides(x) <- valuea SlideCatalog