This function returns the hessian, i.e., the matrix of
second derivatives of the log likelihood with respect to the
argument x.
mig_loglik_hessian(x, beta, xi, Omega)a d by d matrix of second derivatives if x has length d,
else an n by d by d array if x is an n by d matrix
n by d matrix of quantiles
d vector \(\boldsymbol{\beta}\) defining the half-space through \(\boldsymbol{\beta}^{\top}\boldsymbol{\xi}>0\)
d vector of location parameters \(\boldsymbol{\xi}\), giving the expected value
d by d positive definite scale matrix \(\boldsymbol{\Omega}\)