subsystem function?

[rkurchin]

From Zoom discussion today, it came up that having a function to return a subsystem would be useful. Currently, indexing/iteration operates at the level of atoms (or AtomView objects), and I think this makes sense, but there is also utility for getting back out another system object of the same type that contains only a subset of the atoms.

Potential discussion point: should we suggest/attempt to enforce whether things like boundary conditions/box size can or cannot change from system to subsystem level?

I know @tjjarvinen has more thoughts on this, I'll let him chime in with more.