This function returns the gradient vector of the log likelihood with respect to the
argument x.
mig_loglik_grad(x, xi, Omega, beta)an n by d matrix of first derivatives for the gradient, observation by observation, or a d vector if x is a vector.
n by d matrix of quantiles
d vector of location parameters \(\boldsymbol{\xi}\), giving the expected value
d by d positive definite scale matrix \(\boldsymbol{\Omega}\)
d vector \(\boldsymbol{\beta}\) defining the half-space through \(\boldsymbol{\beta}^{\top}\boldsymbol{\xi}>0\)