Get rid of the union particle vector handling in _writesnap_common/_readsnap_common

Export functions currently assume that partricles are the only thing that need to be saved for every snapshot. Due to the growing diversity of particle containers containing all sorts of things, this is no longer true.

My proposal to fix this that all particle containers with non-standard dynamic objects should overwrite writesnap and readsnap (which is easily possible, as they dispatch over the simulation).

@jisense what do you think?