Specifying permittivity with geometric primitives about angler

[twhughes]

Sounds good to me. We could also define our own, seems simple enough.

In a way, what's in structures.py is a basic template for defining complicated permittivity distributions out some geometric parameters. We could also define a more general geometries.py package that allows users to define basic shapes to construct structures out of.

[shoyer]

scikit-image defines a bunch of these: http://scikit-image.org/docs/dev/api/skimage.draw.html

[twhughes]

@shoyer This looks really useful, thanks!

When I get some time, I may overhaul the structures.py interface and use these for convenience.

[twhughes]

@ianwilliamson @momchilmm We can use our shape @primitives for this potentially