A tidied data.table, with columns of 'site', 'y', 't', 'w', 'date' and
'SummaryQA'.
site: site name
y: real value of EVI, [-1, 1]
date: image date
t: exact compositing date constructed from DayOfYear
w: weights
SummaryQA: A factor, QA types, one of "good", "margin", "snow/ice"
or "cloud".