helper function to safely convert a Hessian matrix to covariance matrix
.hess_to_cov(hessian, tol.solve = 1e-09, tol.evalues = 1e-05, ...)
hessian matrix to convert to covariance matrix (must be evaluated at MLE)
tolerance used for solve()
accepted tolerance for negative eigenvalues of the covariance matrix
arguments passed to Matrix::nearPD