Add watershed as a region type

[shansen5]

Initially this can just require the user to enter a HUC-8 code. Think about how to make it more friendly.

[ericnost]

We could try to start with a map. See this as an example:

https://colab.research.google.com/drive/1FFzyAGGWtMpI4S99Y8TH6tByu8aemaox#scrollTo=kSjgi6-YWzo1

(I haven't created a PR for this version of Cross-Programs, but I think it's close)