Returns the Hessian matrix of the log-likelihood evaluated at a given point, calculated with respect to the parameterization in which the random effects covariance matrix is represented through its Cholesky square root.
params : MixedLMParams or array-like
The Hessian matrix of the profile log likelihood function, :
evaluated at `params`. :
If params is provided as a MixedLMParams object it may be of any parameterization.