Detect rowwise grouping

[courtiol]

Users unfamiliar with lay may call rowwise() beforehand.
Doing so should exert no effect besides slowing down computation massively.
So perhaps we should detect rowwise groupings and act upon ithat (remove temporarily, warn, err, ...?).
That would probably imply to detect if the object is a tibble.
We need to see how this is efficiently done in dplyr.