Added APLpy examples

I've now added examples that show how to export the structure contours in third-party packages such as APLpy or DS9. This required adding a method to get the structure for a given ``idx``.

@ChrisBeaumont - this is ready for review, and a preview is at:

https://dendrograms-astrofrog-v2.readthedocs.org/en/latest/plotting.html#plotting-contours-of-structures-in-third-party-packages