two-station methods

2 station methods aren't and can't be a priority in the near future (fall/winter 2016), but we'd love to implement them someday.
- If you'd use a two-station implementation if we made one, please reply or add a thumbs-up to this issue so we can get a sense of demand.
- If you use R and would consider implementing the two-station method in `streamMetabolizer`, we'd love to talk about it and give you credit for any contributions you make!

The issue names streamMetabolizer but no files, tests, entry points, implementation scope, or acceptance criteria. Start by clarifying the two-station method and expected demand with the project maintainers; done cannot be determined from the issue alone.

Written by the indexing model from the issue text.