Multiple particle classes

It would be nice to be able to annotate multiple classes of particles in a single session.

It isn't clear to me the best way to write this type of annotation to disk. Some options:

- Single star file with a column to annotate the class
- One star file per class
- Something else?

Any thoughts? cc @bbarad @alisterburt @kephale

The issue names no files, tests, or entry points. First resolve how multiple particle classes should be represented on disk; done would mean a chosen format and support for annotating and persisting multiple classes in one session.

Written by the indexing model from the issue text.