Implemented multiple selections, linked scatterplot views.

I believe I implemented everything requested:

* Multiple selections using 3 different mouse buttons (for me: click, alt-click, ctrl-click, respectively)
* Scatter plots from the catalog data that are linked to the DendroViewer via a hub (@ChrisBeaumont helped much here) -- this is in dendro_scatter.py
* Cosmetic changes to some of the above, using colorbrewer colors, more opacity, multiple SelectedLabels, and ensuring that smaller/higher-up structures always plot over larger/lower structures
* Rebased into one commit (compare #103, #101, and #100, which have some screenshots and discussion).

The docs haven't been updated yet to reflect the new functionality, mainly since I'm not sure where to make such changes.