Reconsider how to handle groups

[Bisaloo]

This was triggered by @avallecam's messages on slack.

Currently, we state:

https://github.com/epiverse-trace/datatagr/blob/279c0c8d78640d54f547b2fb4778f4f553a06cab/vignettes/compat-dplyr.Rmd#L165

Is this really what we want? Or do we want to preserve labels in this situation? Or do we want to warn when unclassing the objects (rather than "silently" converting them)?

Closing this issue with "the current situation is fine" is an acceptable outcome. I just want to ensure we don't let unjustified historical decisions prevent us from improving the package.