as defined by Xin et al. 2015 (Rem. Sens. Env.)
AGSI(par, data)
a vector of parameter values, this is functions specific
input data (see reference for detailed description), data should be formatted using flat_format()
raster or vector with estimated phenophase timing (in DOY)
The starting point of accumulation is not clearly indicated in the publication so we assume December 21.
if (FALSE) {
estimate <- AGSI(data = data, par = par)
}