Probit model Hessian matrix of the log-likelihood
Parameters : | params : array-like
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Returns : | hess : ndarray, (k_vars, k_vars)
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Notes
\frac{\partial^{2}\ln L}{\partial\beta\partial\beta^{\prime}}=-\lambda_{i}\left(\lambda_{i}+x_{i}^{\prime}\beta\right)x_{i}x_{i}^{\prime}
where
\lambda_{i}=\frac{q_{i}\phi\left(q_{i}x_{i}^{\prime}\beta\right)}{\Phi\left(q_{i}x_{i}^{\prime}\beta\right)}
and q=2y-1