#' Obtain delta forecast by the Persistence method
#'
#' @inheritParams getForecast
#' @inheritParams getZeroDeltaForecast
getPersistenceDeltaForecast = function(data, today, memory, horizon, shape_params, ...)
{
	# Return level of last similar "tomorrow" (thus -6) computed on "horizon"
	data$getSerie(today-6)[1] - tail(data$getSerie(today-7), 1)
}