Store a subset of the particles in the simulation

[ijpulidos]

It should be useful to have a way to store only a subset of the atoms/particles in a simulation. Especially for storage volume usage this might be needed for some use cases where you have to serialize with a high frequency but you don't care about certain atoms/particles in the system.

As far as I can tell, the current state is that the MultiStateReporter is serializing everything in the checkpoint and non-checkpoint files.