Composing annotators with different region_types

[jlstevens]

Right now each annotator only has a single region_type (e.g. range or point) but we need to support cases where both are on the same plot.

With the new compositional semantics, this approach makes a lot of sense to me:

range_annotator * point_annotator * plot

Hopefully this fits in the design in a similar way as multiple annotated plots in a layout.