This function estimates the L-moments of the Laplace distribution given the parameters
($\xi$ and $\alpha$) from parlap.
The L-moments in terms of the parameters are$$\lambda_1 = \xi \mbox{,}$$
$$\lambda_2 = \frac{3 \alpha}{4} \mbox{,}$$
$$\tau_3 = 0 \mbox{,}$$
$$\tau_4 = \frac{17}{22} \mbox{,}$$
$$\tau_5 = 0 \mbox{,}$$
$$\tau_6 = \frac{31}{360} \mbox{.}$$
For $r$ odd and $r \ge 3$, $\lambda_r = 0$, and for $r$ even and $r \ge 4$, the L-moments using the hypergeometric function ${}_2F_1()$ are
$$\lambda_r = \frac{2\alpha}{r(r-1)}[1 - {}_2F_1(-r, r-1, 1, 1/2)]$$