Support a "daylight" step for the unrooted layout algorithm

Felsenstein 2004 has a chapter on tree layout which gives a few examples of tree layouts. One of these is the equal-angle algorithm which Empress (and [Gneiss](https://github.com/biocore/gneiss/blob/d83c4ef7e35c9eaef2c534dc3d00264183286af7/gneiss/plot/_dendrogram.py#L263), and PyCogent, ...) uses for unrooted layouts. This works pretty well, but it could be improved -- there are a few places in the moving pictures tree, for example, where there's unused space (so some of the branches could be angled outward a bit to separate them and take advantage of this space):


Felsenstein describes the equal-daylight algorithm, an extension of the equal-angle algorithm that is apparently efficient and makes even prettier drawings. It doesn't seem like it would be really difficult to implement (... hopefully ...), so this might be a nice way to make the first layouts the user sees in Empress look even nicer.

(If having an extra step in the unrooted layout would be a problem for massive trees, we could probably make this optional, and/or detect the tree size and adjust accordingly.)

Start by locating Empress's unrooted equal-angle layout implementation and compare it with the equal-daylight algorithm described by Felsenstein 2004 and the linked Gneiss implementation. Determine how the additional layout step should behave for large trees and whether it should be optional; done means unrooted trees use the new daylight layout without regressing existing drawings.

Written by the indexing model from the issue text.