Improve IO module

The io module could use the following improvements:

1. The module's name suggests it is for both importing and exporting geometries, but currently it only supports exporting. We should allow users to import geometries from the same file formats, within reason.
2. The module currently only supports exporting from instances of Polyhedron and its children. It would be nice to extend this functionality to Sphere and ConvexSpheropolyhedron. Additionally, we should consider whether it would be useful to extend this functionality to 2D, perhaps by adding support for barebones rendering to SVG.

It makes sense to work on item 2 only after completing #273 , since extending export functionality to rounded shapes requires implementing some kind of tessellation algorithm.