Autoregressive node

ar_unit(::T, order)

Returns [ 1.0, 0.0 ... 0.0 ] with length equal to order in case if T is Multivariate, and 1.0 in case if T is Univariate

ar_slice(::T, array, ranges...)

Returns array[ranges...] in case if T is Multivariate, and first(array[ranges...]) in case if T is Univariate