ngme tensor-product model specification

Given 2 operator (first and second), build a tensor-product operator based on K = K_first x K_second (here x is Kronecker product)

tp(first, second, ...)

Arguments

first: ngme_operator, left side of kronecker model (usually a temporal or iid model)
second: ngme_operator, right side of kronecker model (ususally a temporal or spatial model)
...: ignore

a list of specification of model