Should we have functions to add and remove atoms to/from a `System`? about molly.jl

[jgreener64]

Another consideration is how to make this work efficiently when doing trials in a Monte Carlo simulation. You may want to add atom(s), sample the energy, then decide to keep or remove the added atom(s).

[ejmeitz]

The only reason I can think of to add this is if you want to support the grad canonical ensemble. I'm sure there's other as well, but at the moment I don't see a reason to hack the interface to add this. I think effort would be better put in making the interfaces more generic so that changes like this are easier.

[jgreener64]

Yes, that's the idea.