Mark nest_by() and group_*() as questioning rather than experimental

[DavisVaughan]

• Since we will probably have nest(.by =) in tidyr
• And the group_*() family doesn't fit super well in conjunction with the new dplyr .by ideas

[manhnguyen48]

Hi, dplyr::nest_by returns a rowwise data frame while tidyr::nest(.by=) doesn't. Those are useful to organise tibbles within tibble and not having to use purrr::map on every mutate calls. If nest_by is being phased out perhaps tidyr::nest() could gain a .rowwise=TRUE argument for backward compatibility?

[hadley]

@manhnguyen48 we are considering a different way to achieve a rowwise() since mutate() is really the only function that rowwise() affects, suggesting that creating a custom class isn't the right approach here.