Add hierarchical labeling to data

[peterTorrione]

We can have multiple "Targets" - it would be nice to enforce a hierarchy on the targets - so that plots, displays, etc., knew to group by T1, then group by T2, for, e.g., plots.

In landmines you might have two labels you're interested in exploring - "Lane" and "Mine/No-Mine". It would be nice to say:

ds.hierarchicalLabeling = {'Lane','isTarget'};
ds.plot(); 

and have the resulting data plot use, e.g., color for lanes, and within a color, use shape for isTarget.

The same could be done in imagesc, etc.

The exact implementation details are unclear. Use multiple target columns? Multiple target columns probably breaks a ton of classifier code. Use fields in the observationInfo? Other?