new fork option: new animal (in a group session)

[JonJagger]

Currently, when you are in a group session (say the tiger) you can fork a new session from one of your traffic-lights as either a new individual session or a new group session. Either way, there is no way to find the newly forked session from the session it was forked from. This can hinder reviewing. Suppose the tiger was being worked on by a pair who both wanted to carry on, but individually (perhaps as 'homework'). There should be an option, when forking from an animal in a group session, to create a new animal in the same group.