Function update_hessian_bfgs_pure 

pub fn update_hessian_bfgs_pure(
    hessian: &DMatrix<f64>,
    delta_x: &DVector<f64>,
    delta_g: &DVector<f64>,
) -> DMatrix<f64>

Pure BFGS update without curvature check (MthUpd=5).

Same as BFGS but proceeds even with negative curvature. Use with caution - may produce indefinite Hessian.