I implemented the Lasso select in Scatter. There are still some kinks.

But basically, this is probably the framework that we'll be using!
The main obstacles to tackle:
rethinking how SelectionHub.select() should work, now that we might use the Lasso to select multiple unrelated structures.
The previous selection methods always assumed you wanted a single structure plus all its children, but now we might want to split these into two use cases:
1. You pick an object and want all its children
2. You pick a handful of unrelated objects and you don't want their children

in which case it might make sense to reimplement (1.) as a special case of (2.) which activates when you pass the select() function a single structure (and then it goes and populates the selection with all the children before passing it along to the plots) or something like this. You see what I mean?

This kind of thing would necessitate changing some of the logic in, e.g. BasicDendrogramViewer._update_lines() which calls DendrogramPlotter.get_lines() which has the code ```structures = structure.descendants + [structure]``` implying that when you plot some lines you always want descendants. I'm not super sure how hard I want to fix that, but if we wanted to have a perfect logical correlation with the scatter plots you circle and the specific branches that get highlighted, we would need to circumvent this "get all descendants" logic.
Minor style change: list(blah) instead of [x for x in blah]
Implemented non-subtree selections, so that Lasso select can work.

Fairly substantial change -- I've tested it decently well and I think it works great!
For most situations the syntax doesn't change but in some cases it does.

I think the only thing left "dangling" is that contour selection still only uses the "zeroth" struct, so I gotta use a loop in that case.
Implemented contours to add together for unrelated structures.
Made plot_tree conform to the new signature of get_lines.
Added screenshots to the docs.
Updating documentation readmes with lasso selection.
Added a real docstring to Scatter. Borrowed from an online example.
Addressing Chris's style suggestions.
Fixed bug: crashes when no points are selected in lasso.
Deleted all code referencing widget locks for lasso. Too extreme?

Well, it solved my problem - I don't even know what the locks were for.
Removed completely duplicate callback addition from Scatter
Now instances of Scatter initialize with all pre-selected structures
Added a note about how to see the available colnames.
Added log convenience methods, and noted their existence in the docs
Added note on convenience log functions to the docstring example.
Merge pull request #109 from tomr-stargazer/scatter_picker

Implemented the Lasso select in Scatter.