What to do with categorical values with frequency 0?

[GuyBaele]

When loading a trace with categorical values into Tracer, it may contain several categories that were only visited during the burn-in. When the burn-in is subsequently omitted, some categories will have frequency 0. I've committed a small fix that avoids a NullPointerException for those categories, but the question remains: should we still plot that category (in the marginal density panel) but with frequency 0, or should we not plot those categories at all?

[rambaut]

The other aspect of this is that a categorical value may not be visit even in burnin. Ideally all values should be shown even if zero frequency. Tracer has no way of knowing unvisited states exist. BEAST could define the set of categorical values in the log file's header (the initial lines prefixed with #).