Serverside culling e.g water

It's easy enough for the client to not render water molecules at will, but for large simulations the inclusion of water in frame data is wasteful ~~and more critically might blow over the size limits of our messages~~. Do we want to support some means serverside of culling particles--either by residue or particle index or something else?

On the NoH20 branch this is done simplistically by filtering out atom and bonds above a certain index (where water is partitioned to the back half of the simulation), which is provided as yet another server argument. This relies on the simulation input being ordered in a specific way and a manually provided cutoff number, which seems clunky.

Once option could be to provide a flag or utility function to do this preprocessing and cutoff automatically. Another could be to filter by residue name.

Another axis to this is whether the filtering is done globally for all subscribers, or is something that can be requested at the time of frame subscription (similar to how frame interval is).