Called by the companion package slideimp.extra from their .onLoad()
to register a callback that turns character manifest into a data.frame for
group_imp()/prep_groups().
Value
Invisibly returns NULL. The function is called only for its
side effect of registering the resolver.
Examples
# Typically called from `.onLoad` by `slideimp.extra` package, not by users directly.
dummy_resolver <- function(x) data.frame(feature = character(), group = character())
register_group_resolver(dummy_resolver)