This function is only suitable for 16-day EVI time-series.
init_lambda(y)
library(phenofit)
data("MOD13A1")
dt <- tidy_MOD13(MOD13A1$dt)
st <- MOD13A1$st
sitename <- dt$site[1]
d <- dt[site == sitename, ] # get the first site data
lambda <- init_lambda(d$y)