Possible bug in `clone` logic

[nikhilwoodruff]

Sometimes when cloning a simulation, I've found that the original simulation is inexplicably affected by actions in the cloned simulation. I've seen this for a while but have been unable to generate a MWE, but yesterday I think I found the issue, in this line. When a group population is cloned, it looks like the clone still has access to the original members, which is unchanged.

[bonjourmauko]

Thanks @nikhilwoodruff .

@benjello at first I thought this was a duplicate of #848 , yet reading both issues, it appears like the behaviour described here is intended. Do you confirm?

[benjello]

I think the behaviour expected by @nikhilwoodruff should be enforced.