extended summer metric

[scelmendorf]

@thme3808 @swnordstrom @sean-streich I gave you all access to the NWT long-term trends repository, which has the extended summer metric in it. There is a readme within that folder that explains what most of the things are. Caitlin can also answer more specific question as she made the latest iteration, or lmk if you want me to send you the proposal and/or mid-term review which also has some more deets on what goes into it. Most you can probably see from the PCA loadings, it's things like length of season above some temp thresholds, drought stress, etc.

[swnordstrom]

Thank you @scelmendorf! I'll take a look later this week and see if I have any questions.