Computes the double exponential link function, including its inverse.
doubexp()The double exponential transformation of theta.
Ford I, Torsney B, Wu C (1992). "The Use of a Canonical Form in the Construction of Locally Optimal Designs for Non-linear Problems." Journal of the Royal Statistical Society B, 54, 569-583. 10.1111/j.2517-6161.1992.tb01897.x