two-station methods

[aappling-usgs]

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!