Implement `byfragment`-like selection.

[IAlibay]

Is your feature request related to a problem?

Following up from conversations on discord, it seems that implementing a byfragment (probably better named bymolecule, whole, other?) selection would be convenient. This would avoid the need to expand selections to whole fragments by hand (from @richardjgowers - we might be able to arrange things to avoid copying AtomGroups too).

Describe the solution you'd like

A new keyword akin to byres that expands to fragments.

Describe alternatives you've considered

We don't do this.

[csbrasnett]

I don't think anything was done with this, but I'm commenting to say that this would be very useful for me as a feature. Is there any possibility of implementation in the near future?