Creates an ordinary least squares model with polynomial trend.
OLSModel(degree = 1L, differencing = 0L)A spec for a Julia `OLSModel`, built when it is fitted (see [juliabridge::julia_spec()])
if (FALSE) { # \dontrun{
# Linear trend
model <- OLSModel(degree = 1)
# Quadratic trend with differencing
model <- OLSModel(degree = 2, differencing = 1)
} # }