🧹 avoid unnecessary one_group groupby operations

[jvdd]

Just a quick implementation

[nicolaskruchten]

Wow, that's really funny: I was working out in my head how to do exactly this while waiting for my kids to fall asleep so I could go implement it afterwards! Nicely done, and thanks for taking the time :)

[nicolaskruchten]

Now one_group can be something else than a function maybe ;)